So...

I'm not as certain as you are about their being in love. I do think they liked each other enough, but to the extent of calling it love, I am not sure. It's definitely plausible but I wanted to bring up this other possibility.

I think ch. 6 was less about love than being partners in crime. From the start, the two make it clear that they're people with loose morals and innocence is associated with bad sex skills. The uke says "You wouldn't understand, Kaga-san [...] This kind of pure feeling", to which Kaga-san replies "Like you can say that. Seriously!". They are both impure, and they both know it.
When they meet up by coincidence, the uke says "To think I'd meet you in such a normal place!" is again reinforcing the idea of abnormal vs normal. They're abnormal beings roaming in the territory of the normal, and it's especially true in the uke's mind. He really believes the seme is the same as him and is constantly testing it:
"If you have some time, how about we do it?" - although they met up in a "normal place", the uke won't let the seme be normal. And when the seme says he already has plans, the uke still wants to confirm that he's the same sleazy no-good Kaga-san: "Is it the usual married girl?". It's only after that "test" that the uke reveals he was on his way to a date, which makes Kaga-san wonder why he'd even ask him out.
His asking Kaga-san to put marks on him is a test on his official boyfriend, but it's also a way to keep Kaga-san on the guilty side, by making him actively participate in the cheating.

When they bump into each other again, it's the same location as the first time. I might be pushing things but this can be read as a sign that, deeper inside, both of them have not changed. And the uke really cannot accept the idea of a Kaga-san that is different and no longer impure. He notices for the first time that Kaga-san is honestly trying to fall in love seriously, something he previously thought was impossible, and that upsets him: it doesn't suit Kaga-san, it's disappointing, the uke really doesn't want to see that happening. When they're doing it after the uke's successful attempt at seducing Kaga-san, everything he says is him pointing out that Kaga-san is still no good: "Huhh, six years younger? Really? Fufu" (which parallels the "Don't play too much with youngsters" from earlier), and "She must be [cute]. You've always been about the face after all", implying the seme didn't genuinely fall for his girlfriend. And then there's the "You don't have sex that much?" and "You're still amazing, Kaga-san", which reads to me as "even though you were in a serious/pure relationship, you're still the impure Kaga-san" as purity is associated to bad sex. Finally, the line that needs no explanation: "I'm relieved to see you're still the same good for nothing, Kaga-san."

I'd also like to point out the title, "Fellow Traveler". According to Wikipedia, the term "identifies a person who is intellectually sympathetic to the ideology of a political organization, and who co-operates in the organization's politics, without being a formal member of that organization". The Japanese title, "道連れ" (michizure) can be translated directly as "fellow traveler", or "someone unwillingly made to take part, being made to take part in something". http://jlearn.net/Dictionary/Browse/1454280-michizure-%E3%81%BF%E3%81%A1%E3%81%A5%E3%82%8C-%E9%81%93%E9%80%A3%E3%82%8C
I think it's a particularly telling title; more than lovers, they are partners in crime.

This is a good theory and you do bring up some good points here with the parallels and the hidden intonations. Personally, i still stick with my own theory (sorry, there just wasn't really enough here to sway me : / ) but you clearly put a lot of thought into this and it is definitely a possibility. To me, the fact that the uke was so willing to have sex with kaga at any point in the story is proof that he was never really invested in his new seme and if he wasn't invested in him then why would he talk to kaga WHILE having sex about another guy that he is supposedly getting serious with... to me this is clearly him trying to make kaga jealous, but kagas dense. It really seems evident to me that kaga's dense when he says the whole "i'm gonna miss him" line when he's drunk, but doesn't acknowledge or realize he has some kind of special feelings for him. Hell, the fact that he has sex with him even after having gotten into what he himself views as a serious relationship points towards him having feelings for the uke, but i guess it's all up to interpretation. Neither one of us has been proven right or wrong and both are definitely a possibility.

Oh I definitely didn't intend to prove you wrong or anything! I just wanted to point out my thoughts but I think you are just as right as I am, and tbh both theories aren't mutually exclusive. I'm sorry if my tone was aggressive;; It's the effect of having too many papers to write for school-- (hesitation doesn't sound too good in academic papers)

I totally agree about the opening being about making Kaga jealous by mentioning the other guy while they've having sex. The only reason why I didn't take that as the uke being in love for sure is because to me, it doesn't have to be about love? And same for Kaga giving in at the end (I guess this is really a personal opinion because I'm basing this off... personal experience...... *sweats*)

But this exchange has been fun to me! Thank you for actually reading and answering my long wall of text!

Oh you're fine, (づ￣ ³￣)づ I didn't feel attacked or anything, i was just responding to your response. I really did like your theory and did think that you put thought into it which is why i said so. (⌒▽⌒) You're exactly right on what you said in that these are both just possibilities based solely on our own interpretations of what happened which was what i was actually trying to say as well in my previous response when i said "these are both possibilities and neither of us has been proven wrong or right", if any of my response came across bitchy or confrontational/defensive it wasn't intended to; it can be hard to communicate through text. ╮(￣▽￣)╭ I was essentially saying "great response, i like your theory and while you DO make some great points that i'm not sure i could even debate without proper time, i'd like to respectfully stick with my own theory and keep yours in mind. When i made my own theory it was like a crap ton of blocks fell into place for me, i had been confused as to why the uke acted the way he does and when i realized that it could be because he was acting the way he does intentionally and looked back on the instances that confused me they were easily explained by the idea that the uke was actually in love with the seme and trying to manipulate him." The seme reciprocating this love was kinda a side point to the theory based solely on the drunk scene and can be interpreted as mere fondness, i just like to look further into it.

I think he's just a prick, lol.

yes, i believe he's definitely a prick as well, i also believe that the uke was the first person he started to build an attachment to as well though.

If you meant the uke, then i suppose it's possible he's just a prick, but whats the fun in theorizing if going with such a simple solution?