It's not as if Frank hasn't had a boyfriend or a girlfriend before, he has had many: the longest lasting a week and a half, and that was certainly an achievement. Frank has fucked a lot of people, and that's how he likes things. No relationship, no calling them the next morning, no dates, just sex and then they stop talking and that's that.
Gerard was more of a going on dates, falling in love, getting married, relationship kind of guy, he hardly ever had one night stands because he viewed sex as more of a special, important thing. He wasn't a virgin, mind you: he'd fucked a couple people, been fucked by a couple people although most of them where his previous boyfriends and girlfriends rather than strangers he met in a bar.
They're incompatible, right?