It kinda feels a bit more natural with Gale, since you have 2 scenes that have romantic stuff (to some degree) + and those unlock some new dialogue options, that again, imply romantic feelings. And then you get the camp party scene where he propositions you. So that's that.
Astarion and Lae'zel, as people already pointed out, seem like they'd be into casual sex. Astarion in general is pretty flirty, so it fits.
And Shadowheart only has a kiss, so it's hardly a big deal.
