It does commonly happen indeed, in fact i began to realize i was gay when a straight friend had invited me for a JO session but I found that I was more interested in him than the porn that we were watching -- he realized it and never invited me again, as me looking at him too much made him really uncomfortable. Around the same time (this was all before I was out) I jerked off with another straight friend, but it was under the covers so I couldn't look. Now, even as an older adult, I have straight friends who admit to liking jacking off with other men, but they won't invite me cause I'm gay and that's awkward, so it's obviously just for the straight boys.
I don't find it strange. I certainly wouldn't at all be uncomfortable with masturbating with a woman. Hell, maybe I should find a female jack off partner.