He was pinned aginst the wall....withi his hands being FREE. And he just clung onto that guy, instead of really resisting. So yeah, Tom is cheating. He knows that he's strongger, and if someone really doesn't want something, then they don't just stand therre, they try to fight at least. And not with just a slap.. But he was already thinking about the other guy while having sex with his bf, so I don't think that he really didn't want it in the first place. And what if he slapped him twice? Then he should keep punching him if that moron can't learn the lesson. He's the stronger one.
a kiss can be quickly stolen that's right
but getting hard by that guy's hand means that he's reacting to him, feeling good and getting turned on (u don't just get hard/cum quickly, especially if u're forced)
getting hard isn't just a physical thing, a bigger part is mental stimulation

Everyone needs to stop blaming Tom for this. He never gave his consent, so this is all because of the of the other guy.
"Well he didn't fight back"
He's already slapped the guy twice, and now he has been pinned against the wall. Under normal circumstances, maybe he could have broken free. But because the ass-hole was touching his dick, he was probably in a weakened state. Rant session over.
Oh and spoiler alert........................
It will get worse before it gets better.