i do not believe that won intended to ra** seonho.
looking at won's character, it seems he's very child-like. he's simple in emotions and does not understand how humans interact. he doesn't even understand how much humans need to eat to survive.
won has never had s** before. i don't believe he's done it with a woman either. he has no interest in s**. but for some reason, he has an interest in seonho.
i think he wants to be close to seonho and to possess him, to make sure he never leaves. however, i do not believe that he has any ill intention seeing how emotionally simple he is. any ill intention he could have probably does not mean to be as insane as it turns out, because he truly does not understand. he finds everything interesting and fun like a toddler.
also he enjoys having s** with seonho and obsesses over it because it's his first time feeling such pleasure. as long as he gets what he wants, he'll probably do whatever you ask him. his character could easily be used and abused.
seonho has a strong and violent personality. even before the apocalypse, he was violent. he's also extremely stubborn. even though he agreed to everything, it doesn't stop him from lashing out. even if he was dying, he wouldn't ask for help. but he's also very timid and soft hearted. which is why blondie can be emotionally domineering to him and seonho is okay with it. seonho can let go some of his stubborness to help blondie.
i think, more than the ra**, seonho is bothered by won knowing how timid he actually is.