A friend with benefits asked me how I would respond if he asked me to be his girlfriend, why would he do that?

Wake up!
He is manipulating you and you are falling into the trap. He is obviously only interested in getting into your pants and nothing else.
It is so obvious that you should have realized it the second he told you that he does not want a relation. That is a typical behavior of a player. To get into your pants yes; to commit, no.
Realize that you are just a number on his trophy list and just another victim to add to his body count.
Get rid of this guy because he will only managed to confuse you even more and to misuse your emotions until you don't know what is real and what not.
Opinion
3Opinion
he wants you to be his girlfirned its very simple. all friends with benefits end this way
Oh thank god, I’m not weird. I’m not the only one. Well, I wouldn’t be the only one.
Why do you think? Are you really this stupid?
Superb Opinion