Do guys get attached?

I've know this guy from my work nearly a year and we get on really well, i'm pretty sure he fancied me most the time we've been working together.

Lately things have got very mixed up - and i know what i've done has been wrong...

He has had a girlfriend for nearly 5 months now (i think) and it hurts a lot - especially as the whole time he's still acted into me and has said that he really liked me and all and whenever i've confronted him about the acting like he's into me thing, he's said something like "i do it cos i did really like you" and said that he liked me and her about the same before him and her started going out (not sure whether to believe this).

About a month ago we ended up sleeping together but then he got all distant and then we kinda agreed it shouldn't happen again

then at work today he strongly suggested that he wanted to do it again and said how his gf had cheated on him and has "two friends with benefits" and was like "owww why cant we be friends with benefits again?" and stuff.

I have a very hard time saying no to people who persist and ended up giving him a b.j... i know i shouldn't have but like i said, i find it hard to say no, especially when people are persistent and i really like them so am planning on telling him that what happened at work today shouldn't happened next time i see him and if he asks why i'll tell him that i'll get attached and would start wanting a boyfriend, not just a quickie every now and then. (good idea?)

I know fwb only really works when you're not attached so i know me and him could never really be friends with benefits when i'm already attached to him, but what is the likelihood that he'll ever be as attached to me as i am to him?

I'm unsure because i guess i've been pretty "easy" or whatever and he has a girlfriend but then again, we have been good friends for a year and his gf has cheated on him? or does he just wanna get her back?