Hmm... I wanted to learn, so never thought about it too much, but I've taught difficult students for several years, and so some of the following might be worth trying. Forgive me if it is out of place.
You might try an old teaching trick... teach them the signs that are most important to them. Someone mentioned ILY. You might also try things like, "Would you like..." (to go out, a kiss, etc). Share these signs in a private environment, and at times that you are both feeling very good about each other and yourselves. ie if you joke alot, you can sign "funny!!" "you're crazy!
" and other words in addition to another mode(s) of communication. Also, don't correct them too much at the beginning, let them get comfortable with the idea of signing first.
You might also try a trade... if he wants you to learn sky diving or whatever that he enjoys, you can learn sky diving while he learns sign... if that is the trade, though, when he is fluent, then you have to jump, so be careful!
Also, I am hearing and so is my bf, but I tend to flirt in sign language to some extent. Hmmm... he doesn't seem to mind at all!
Also keep in mind that there are reasons that people don't want to learn new things. There are many reasons, including that they may be: intimidated that someone knows something they don't; afraid they'll make fools of themselves; afraid (especially with something as visually open as sign language) that they will call unwanted attention to themselves (or to you as a couple at a time when they would like some privacy). Beginning signers are often extremely aware that "everyone's watching" and "I'm no good at this!!" If you make them feel like their signing is as romantic and sexy as a few phrases of French, they might decide its well worth learning!!
