I love masturbating and am very turned on by watching other people (men and women) masturbate, so one requirement I've always had for my boyfriends is that they be willing to play with themselves for me. It's an incredible turn-on for me to see guys stroking themselves, especially those last half-dozen strokes when they start coming, something about seeing that sticky white cum pushes me absolutely over the edge.
But I have an even bigger turn on, it's watching guys tasting their own cum. I love the idea that a guy is so horny that he'd suck his own cock or taste his own jizz, and I've spent many many hours masturbating to this fantasy.
After breaking up with one of my boyfriends I decided to add a second requirement (the first being that my BF has to wank for me), I wanted to see him eat his own cum for me. I know guys love it when girls will taste/swallow, so I made a deal with my new boyfriend, he could cum in my mouth as long as he was willing to taste it first (!)
Surprisingly (or maybe not), both of my next two boyfriends agreed. The first BF was flexible enough to be able to lick (but not suck ... darn) the head of his cock, and I will never forget watching him stroke his shaft while his tongue circled around the head of his cock. When he finally blew his load onto his lips, I thought my heart would stop with excitement. Unfortunately, the relationship didn't last and I could only get him to do it once, but the next BF was much more accomodating, he couldn't get his cock to his mouth, but I was willing to settle for him lying with his legs over his head (plow position, for you yoga people) so he could aim his jizz at his waiting mouth. It took a few tries to get it right, but soon he was able to drop pretty much his whole load onto his waiting tongue. What's better, he was willing to do it on a weekly basis, and I rewarded him by squeezing his balls dry almost anywhere and anytime he wanted.
That relationship ended too (but not before I had watched him eat his cum probably a hundred times!!!), and I decided that it was better not to try my special fantasy on my last BF, now husband, but I still masturbate to those memories a lot.
So if you share my fantasy, don't be afraid to offer your BF a deal, you eat it, and then I'll eat it, it worked for me!