You wouldn't be feeling that way if you devoted your time, love and welfare with someone only to find out they were cheating and have sexual relations with 4-5 other people behind your back.
It's all about perspective, hun, and tbh as a trauma survivor on multiple fronts someone like Masami being in a relationship with someone like Tora just reeks of toxic co-dependency and is not healthy in the least.
Do they love each other? Yes. Probably more so than we think. Is it healthy? No. So far we know that Tora is a trauma survivor and has never sought out actual help for it. He literally thought Masami was someone sort of pure angel till he found out that-- surprise, surprise!-- Masami was actually garbage. But! He chose to cling onto Masami 'cause he was the first human to ever show affection towards him.
It's the same thing with Masami. We don't know what sort of trauma he is hiding but it's obvious that he hasn't sought help if he's turned to habitual cheating. It shows that he is unhappy, unsatisfied or scared of something happening to his well being so he puts himself first before others and seeks out whatever pleasure he wants regardless of the risks (we see this play out with the abusive psycho. There was literally no benefit to that relationship other than the sexual gratification he got from sleeping with his boss.)
It's not cute, brave or understanding that Tora put Masami's needs before his own. It was self-sabotage or martyrdom if you want to romanticize it for whatever reason. Masami may have some sort of past but as far as I'm concerned he's got a LOT of growing up to do before at least myself to see him as a proper partner for ANYONE.
I really like this manga and look forward to seeing updates. I'm not mad at Masami for having multiple partners at all. It's something he's honest about but not necessarily proud of. I get that people might be upset at this lack of good moral judgement but.... that doesn't take away from the fact that otherwise Masami is a caring, generous sweetheart.
There are many things that could've caused Masami to begin indiscriminately cheating. I feel like most of his other romantic relationships are in the pursuit of sexual gratification and praise. Even when he finally realises that Tora is all he needs, he's only hesitant to change their relationship out of fear of the unknown...of being discarded. He's afraid of being alone and I think multiple lovers are just a safety net. Can't wait for more development!