Not OP, but probably realistic is a better word for it. OP mentioned nerdy, so maybe physically weak, shorter than average, etc. Basically the opposite of 6’+ gym rat with year round 6-pack.
I'd prefer if the protagonist did have other typically nerdy traits (a poor social life, an interest in technology, movies, literature etc.) but I wouldn't exactly mind a more realistic protagonist since I don't have many choices.
Kind of an odd suggestion, and I know this might not work if you’re someone who wants to place themselves in the story, but if you’re looking for healthier, better adjusted men in literature, try some queer or gay romance. Not sure exactly your desired age/reading level, but Red, White, and Royal Blue is cute, and while socio economic status makes the main character slightly less relatable, it’s still a nice read. There’s plenty of other excellent queer romance that has well written men. If you also enjoy fantasy and political intrigue, Priory of the Orange Tree has excellent world building and a few romance subplots I think are very sweet.
EDIT: Somehow missed the “nerdier” part. Let me do some library diving and give more accurate suggestions
I haven't read a lot of romance novels honestly but I read 11/22/63 last year by Stephen King. At its core its a historical fiction (very long read, I did audiobook) but there's a love story interwoven and I found it very romanic. The main protagonist Jake seems very average to me and his views on life and the world were very relatable. Just an English teacher trying to make a difference in the world.
I don't know where in the world you are so there may not be a similar service for you, but the Brooklyn public library has Book Match. You tell them what you're looking for, and a librarian makes you a list of reading suggestions. I used it once and it was very good.
I can't say such a book doesn't exist, but it might take some searching. Anyway, I recommend Cathy Yardley, who writes mainly nerdy characters. Role Playing is a good book with older gamer characters. I'm also really fond of Attachments by Rainbow Rowell.
I don't remember if it's specifically nerdy but Small Change by Roan Parrish rates very low on toxic masculinity.
Any subgenres you particulary like or anything else to narrow it down?
I don't know about nerdy enough but I found "The song of Achilles" by Madeline Miller so romantic!
We all know the story of Achilles of course, but I found this book very refreshing because the one speaking is Patroclus. Beautiful ending.