At my workplace, there are probably 30 or 40 women I come into contact with during the course of my job, out of maybe 60 or 70 people in my area. One or two are over 50, and hence not very attractive to me (I'm 40). One is probably over 400 lbs, and I am not especially attracted to her sexually, but as she's a great person otherwise, she may be the only example of a woman I get along with at work with whom I would not have sex (were I single, etc...). So, I'm sexually attracted to all these women, though of course I have no way to know which are attracted to me -- certainly I hope they have no way to know that I'm attracted to them... it's the workplace, after all.
So, my experience bears out the author's assumption that if circumstances permit, sex would almost always be "in the way" of a friendship between two mutually possible sexual partners. I can only imagine how much more this must be a problem for those who are bisexual.