Coincidently, it is very close to the population who has watched “When Harry Met Sally”.
Big assumption 
I’m more flabbergasted that you and I see something the same way.
Honesty, sincerity, compassion. Talk.
It may not work, and be prepared for that. It’s up to her to accept his request to dial it back.
But, talk. If the relationship is worth it to him to save, then he can try but it may not be easy. She may not accept the change well.
Talk. Find a quiet, private place, a neutral, somewhat public place. And talk.
In college we had neighbors who had some issues with mismatched sex drives. She was more interested than he was. It was a big problem for both of them because so much of society’s messages for gender norms are that men are always horny and women are always sexy. So if a man isn’t horny, then it must be because the woman isn’t sexy. It strikes to the core of a lot of women’s self-respect and identity to be seen as desirable by the people they desire(not by everyone, in all circumstances, there need to be boundaries) but once you’re in the boyfriend zone, to unilaterally exit the boyfriend zone and head down the hallway to the friend zone, that’s a tough transition. Feelings of inadequacy and guilt were what our friend went through and while they eventually struck a balance and got married, it was a very rough time. He was very sweet about it, reassuring her that he cared about her and doing things for her, etc. But it wasn’t easy, and it wasn’t fast. If this woman is important to you, tell her that, and tell her that in ways that are unconnected to your physical intimacy. Ultimately you may not be able to salvage the relationship, and if not that’ll probably be at least as much society’s fault as yours.
Enjoy,
Steven
OP’s guy: Write a performance piece explaining why she’s too good for a flabby, diseased wimp like you. Then move to another state and order plastic surgery. Consort with orangutans. Become a uTube star. THEN she’ll be sorry!
Genital Herpes, I reckon.
Is Syphilis still a incurable after a certain stage?
I know several younger women who’ve made the transition easily with mutual friends/acquaintances. Or at least it appears that way. Sometimes one or the other finds a more serious partner and/or they just sort of fade away but remain friends.
I think a lot of the responses are over dramatic and too absolute.
“Honey, there’s something you need to know: I’ve got a really bad case of jock itch.”
Cripes no. That is not a lowest risk path. People famously, notoriously, remain in love and attached to people who treat them badly. Now you’ve just got a crazy needy person who you hold in contempt and treat with inhumanity.