So you are asking us about our thoughts on yaoi?

Sorry, couldn't resist. I agree with arachnes_web that these terms are rarely applicable outside of actual yaoi works - uke and seme are terms tied inextricably with a kind of fetishization of gender roles and penetrative sex that mostly doesn't apply unless you're writing a particular kind of kink. I think it's pretty much impossible to speculate how two fictional characters would actually behave in bed because their behavior outside the bedroom doesn't necessarily correlate with their behavior in it and therefore it almost always comes down to person preference. I tend to like pairings for a certain equal give and take, an adventurousness that even extends to the bedroom, so in my case very few pairings I write would fit into such rigid roles.

If you're looking for dynamics that have some similarities to uke/seme, maybe you can check out some BDSM fic - the concepts aren't the same, but there are often more clearly defined roles for each partner.