Now I have forgotten where I read that ... anyway here goes: a theory says that man like to
watch lesbians because women do not have a
refractory period, that is to say, a built-in delay between
orgasm and the return of interest in sex.
Basically, any porn that includes a guy fucking and then coming kind of implies that it will end - and the average guy is only too familiar with eiaculatio praecox.
Lesbian sex (again, in the fantasy porn world) is more of a continuous, possibly endless, activity. The sight of two lesbian sex kittens entwined and engaged in happy mutual cunnilingus or whatever it is that lesbian sex kittens do in flics, does not imply its own end.
And, at least in fantasy, men would enjoy infinite sex.
I am not entirely convinced, but it is a fun theory. And to answer to Hermetic above: for me, it was love at first sight. I did not need to be told that that was hot.