First, I'd suggest going to Groups and joining Michelle's Writers Group, which is a help spot for new writers and new to Lush writers.
As for the themes. They are all rather tried and true variations. It is always the writing that makes the story, not the theme. A good writer can make an old chestnut dance; a bad one can kill the best idea in the first paragraph. That said, certain categories are more popular than others -- , , bsdm, for example. That may influence the slant you take, but again, it's all in the words on the page. None of those do a thing for me, but if the author snags my mind, I'll read it to the end.
Okay, sermon over. I do like the Couchsurfing Fantasy (my profile should answer the why), but if that's where you go, I'd want a REAL fantasy, something odd, unexpected, not just fodder for jacking and jilling -- that would be easy and frankly boring.
Welcome aboard
Ps
Be extremely careful if you pursue A Student Abroad. *might involve submission, a little force* is a sensitive topic for many of us and is looked at very carefully by the Mods who vet the stories.
Don't worry about any of that. Just write. Write 'em all.
Every single word you write makes you a better writer. So write as much as you can. You've got three ideas, you've got three stories. Four if you go for both options in the third.
The challenge for you will be in making each of those three or four stories engaging in their own right, given they're all a very similar theme - someone needing to stay with someone else and having sex with them. I would argue a very valuable exercise for a writer wanting to develop their craft.
As I say, don't worry about what's popular. Don't worry about what other people think. Just fucking write! Then write the fifth one, and the sixth, and the seventh, and so on. Learn from each one and make the next one better.
Best of luck.
My latest story is a racy little piece about what happens when someone cute from work invites you over to watch Netflix and Chill.