As long as you're not crashing a serious relationship & everyone knows the score, what's the harm?
I agree with this. Hell, I dated up to 4 women at a time, but I found 4 to be a bit much. 3 ended up being my limit. I started dating my wife when I was already dating another girl, and flirting with many others.
I say go for it. If she's completely dedicated to her boyfriend, she'll let you know. Who knows, she might be wanting to get rid of him and try something new!