What could I use as a thesis for I Corinthians 7: 2-5?
The most likely thesis for this passage would have to do with what Paul thinks about sexuality. In these four verses, Paul is telling us that sex between married partners is a good thing. Your thesis could focus on the idea that people often feel that Christianity is opposed to sex. You could talk about how this passage proves that God wants his people to enjoy sex so long as they do so in the proper context.
It might also be possible to talk about sexual equality in your thesis if you would prefer to write about that. These verses portray sex as something that both husbands and wives should want and should enjoy. You could argue that this shows that God does not see sex as something that is only for men to enjoy.
Either of these would be a good topic for a thesis on these verses.