Roman started compensating for his lack of sexual experience at a young age. When he hit his teens and his peers started bragging about the amount of pussy they were scoring and even shy, stuttery Kendall seemed to be getting laid on a regular basis (probably because of Stewy's influence, but still). He had joined in with the lewd talk, assuming that the experience would come in time. The more time passed, the wilder his stories got.
It's not that he's not interested in sex, he thinks about sex constantly, it's just that when it comes down to actually doing it he freezes. He's had girlfriends, the Roy name alone is enough to attract women as arm candy, but he's careful to keep them on a rotation of a few months and hopes they don't compare notes.
His longest relationship was with Grace, and that was mainly because of Isla. You're such a good guy, Grace said after watching him play with her daughter, and his heart swelled (unfortunately that was the only part of him that swelled) because literally no one had ever said that to him before.
He proposed to Grace that afternoon, and got a maybe in response. He even suggested that he could adopt Isla. Ken's daughter is adopted and he's seen how much he loves her. It could work. But Isla still has a biological father, albeit one she barely sees, and he doesn't love Grace, in fact he finds her breathy voice quite annoying. On the plus side she didn't seem to mind the lack of sex thing, until she hit him with it during their breakup at Thanksgiving. Fuck you, you got plenty.
He proposed to Tabitha at Shiv's wedding because his sister was getting married and they've always been competitive. Kendall has already reached the divorced stage, nearly, and even Connor is 'going steady' with Willa now. He can't be the only one left behind. Tabitha just looked at him pityingly and said Do you think this is the way to get someone to stay?
So being consumed with lust for one particular woman is a whole new experience for Roman. He thinks about Gerri constantly, her voice in his ear, scolding him, her composed demeanour in meetings when Roman knows what she's really capable of. The way she bats off his sexual remarks with an eye roll and responds to his dick pics with thumbs down emojis.
She's the one. He's never been so sure of anything. And the jump from phone sex/sexting buddies to husband and wife seems like the next logical step.
It's only when he asks her Should we get married? and she looks at him like he's grown two heads, that he realises he's got it wrong again. He reacts in the adult way, by making a joke about cannibalism and storming off. He can't forget the look on Gerri's face. He feels humiliated, and not in the good way.
If she asks he'll say it was just a joke, like most of the things he says, but she doesn't bring it up again.