It’s not really a romantic relationship without sexual compatibility (and/or lack of sex)—that’s a platonic relationship. I know it’s really rough to love someone a lot and to know they’d be a great parent, and to want to hang on to them because they’re your best friend. I’ve been there.
The problem is, as you said, when the ‘spark’ isn’t there, I think it’s human nature to wonder if there’s something else out there. I think I was in a very similar situation to yours, and I would see couples together who were very obviously in love and very attracted to each other, and I would get to missing that part of a romantic relationship.
Not only that, but I find it a lot harder to accept someone’s quirks and the things they do that annoy me, when I’m not madly in love with them. Without that spark, all the irritations just build up and it doesn’t make for a fun friendship, much less anything else.
It’s a tough call and I think only you can decide which way to go. Would you be happy spending your life with someone you just don’t feel that extra ‘oomph’ for, but at least have a rapport and mutual caring, or would you rather have that romance, the spark, and with it, some really hot sex? Keep in mind that just because you let the one go, that doesn’t mean there aren’t others out there. You could find someone who’s your best friend, would be a great dad, and also really do it for you in romantic and sexual terms.
I think the point you made about the lack of said hot sex, in that it may make you wander, is pretty valid and I happen to agree with you.