if you haven't lived out your wild side yet, and you're
27...then you're right-- you should probably do it now, before you hit 50 and sink into a midlife crisis.
do it now so you don't have any regrets.
if your girlfriend is truly in love with you, then she'll probably wait on you... but you can't break up with her, and expect her to be there when you decide that you're done with the single life.
(i know that if
i were in her shoes, i wouldn't marry a guy who still had the desire to "experience" other women...)