I guess these things work a little differently for every single person.

I enjoy watching lesbians since I can imagine myself joining them as the man who gets treated well by both.

And as I´m imagining this threesome of mine, the other man lacking in the video for you always somehow seems totally out of place to me. I can only watch the video without him.

But as for all of these sexual tastes and preferences, there is no explanation; it´s just there or it isn´t.