I agree with some things you said. You're right!
I haven't read this story, I was looking at the comments to see what people thought. But I need to answer yours because it's killing me.
You don't need sex to have a romantic relationship the same way you don't need romance to have a sexual relationship. If not being able to have sex make romance pointless to someone then that person was never interested in romance to begin with. They're probably just aromantic.
That being said thinking "without a penis you can't have sex" is really, really dumb, there's a lot of ways to pleasure someone without it. Even my neutered cat find ways to masturbate, I'm sure two humans together can figure it out too. He still have fingers and his prostate and still can't find a way, it's a skill issue.
Whatever people say, if any man got isekaid and then turned into eunuch they would resent their situation too. Having sex is not necessarily the most important part in life or relationships. But not even having the option must be painful, especially for MC who got pretty much everything else like money and power, probably even women, except he can't do anything with them.
And let's be real, most of the people who read isekais want a fantasy romance story, but MC being an eunuch destroys any possibility of it. Romantic love is pointless for MC since he can't do anything with it, so whatever love he may have for someone else probably would never be romantic, but platonic or familial. It's not like people can't live without romance, but for in isekai protagonist having "No Maidens" is embarassing, plain and simple.
Maybe he doesn't need an entire harem, but otherwise his decision to travel back in time to finally have romance in his life is perfectly understandable.