A girl must feel jealousy every once in a while to stay in love.
Women are not turned on by loyalty. They aren't even impressed by it honestly.
Logically you think they would be, because you are. The mistake you made was assuming that male and female psychology was at all alike. Silly man, luckily you clicked on this thread topic.
A man will always value the loyalty of his women because her integrity is the only way he can successfully pass on his genes. Whether thats figuratively or literally.
A woman can only get pregnant once at a time, if she's involved with mutliple guys or cheats on you, your inner animal goes berserk.
Women are not men though. They're only seeking men of high perceived value to fertilize their egg, except for when they're seeking providers, which you do not want to be of course.
Loyality ultimately matters very little to them then. As long as they feel their needs are successfully provided for, your promiscousness is unlikely to cause them much stress. This is why women so often go back to men who cheat on them by the way.
The quickest and easiest way to signal high value to your lady(ies) of choice then? Be involved with other women.
When your girl becomes aware that other girls like you without you trying to get them to, you'll find that the sex has never been better.
Does that mean you should cheat on your girlfriend? No. However, armed with this knowledge, you now have the ability to change your approach.
No more declarations of undying loyalty, which dry up every vagina within a 20 mile radius. No more determination not to be social with any woman at clubs or parties because "you're a taken man". No more rejecting friend request on facebook because you're scared your girlfriend will find out and dump you.
You possess the spirit of Man! Be free to explore the world around you, observing all the options at your disposal, thus making her work hard to keep your attention.
This is how it should be.