Firstly, I just want to get out of the way that in case it wasn't obvious, if a guy loses interest when you deny him sex then he's not worth it.
Anyway, I understand that some women just never really played around with themselves and so never acquired a desire for such things. Some people just don't have any interest in sex at all.... ever. Men on the other hand.... let's just say that at times we're compelled to obtain it much the same way you are compelled to eat food when you are hungry. Sexual frustration can leave a person with a very bothered day.
Now as I stated to begin with, if a person loses interest because you won't touch their penis that's stupid and you should drop them fast. They should respect that you don't want to engage in sexual activities if they really value the relationship. On the flip side, it can come off as very mean to a person who really cares about you. For most people sex is a very emotional bonding experience, and personally if I'd become very close to someone (let's say we'd been together at least a couple years) and they just didn't want anything like that I would be very hurt. I would wonder if something was wrong with me or the way I treated them. I would wonder why we couldn't be as close as possible despite our time together, and most of all I'd wonder if there was something she needed that I wasn't providing in the relationship.
I've never been a proponent of sex for the sake of sex, since it wouldn't be special. Still, if you become serious about someone I would recommend doing it anyway, even if it's just for them. It sounds weird, but it's like playing their favorite video game with them even though you don't particularly care for it. I don't even think you have to go all the way. Just show them that you understand and tend to their needs, just as you would do for each other in other areas of life. Again, only if you think they're worth it.