Some people may be concerned that casual sex is not worthy of being included in the story and that we can headcanon it if we really want to... but... I can think of several examples where this would fit into a story. How about Morrigan's Dark Ritual in Dragon Age? It's not like she was in love with Alistair, Loghain or possibly the male warden.
I'm not asking for something meaningless like the "romances" in Saint's Row 4, which was probably just a spoof of BioWare games. It boils down to "Wanna ****?" - "Sure!" and that's it. In the end the "relationship" felt just as close as in Mass Effect, cause the consequences were the same: none whatsoever.
A relationship that only consists of three conversations and an one-night-stand doesn't get me invested emotionally, so why not just skip all that and go right for the climax? A good writer could make that work.
What I really, really want is a meaningful relationship. Something that's not just one big thing once, but many little things throughout the entire game. Something like a short cutscene that shows my LI and my character having lunch together or I don't know... actually spend time together outside of combat. I never played the Garrus romances but I'd kick that guy out the airlock if I wanted to talk and he told me, "Can it wait? I'm in the middle of some calibrations." No, you're in the middle of finding a new partner. I also wouldn't have minded to spend more time with Kelley during ME2. The stuff after the Suicide Mission? That's game over man, game over!
So, since that is not going to happen... let's just cut to the chase and have fun. No strings attached.