Agreed.
I'd have to say "No." Don't be ashamed. I can only speak from my personal experience that beating yourself up over it can lead to depression and slide into dangerous behavior (alcohol abuse, etc.)
I am Bi. It took me years to accept myself and not feel guilty after "hooking up" with a guy. Mind you, I had a couple of buddies that let me "do things" to them, but then came "that guilt" afterwards. That guilt is society's doing, not my own. If I recall, I enjoyed it and so did my friends, so who cares, haha.
If I am ashamed of only one thing it's this: that it took me so long to accept the fact that I like girls AND guys. I could have probably been happier a lot sooner in my life. Having said that though, I might have had a "double whammy" to deal with, being HOH _AND_ "A fag" in a small town...maybe I had to get over myself first before I could love myself enough to say "Screw it, and screw them"? I don't know. I just feel that feeling guilty or ashamed is a waste of good time.