...oh hi? Hey! She, not he, but sure, I can give this a go
So, this?
Yeah, this is a perfectly normal, real-life thing. It's called romantic attraction.
Most people have the same sexual and romantic orientation, like biromantic bisexual, but some people - often asexuals - have different ones. I'm panromantic asexual. Panromantic is like pansexual, but romantically instead of sexually; I can be romantically interested in someone of any gender. There's also aromantic, where you don't feel romantic attraction, and you can be aromantic and asexual (aroace), or any other combo of sexuality, like aro + hetero or pan or whatever, even homosexual heteroromantic. All the crazy combos! Does that make sense?
Basically, Rlain can be ace and also homoromantic. Still awesomely gay, still perfectly capable of being interested in Renarin. From how Venli treated her once-mate, I've been working off of the assumption that she still felt plenty of love for him, just no particular desire to do the do.
This also wouldn't be a huge leap from Jasnah, who's confirmed heteroromantic asexual. She's not sexually attracted to Wit, but is still comfortably in a relationship with him.
So if Rlain wanted to spend nights with Renarin in mateform and just not make babies, he could, though I agree that singers mostly seem to not be interested in sex for its own sake. They could always adopt if they wanted a kid. Same with Kaladin and Leshwi if Leshwi doesn't transition; having kids isn't something you have to do to be in a relationship.
I hope this has helped you!
Wait, Venli's aro? I totally read past that if so, neat!