I think for men sucking on a strap on it provides a free of guilt "homosexual" experience. I do not think it makes you a homosexual, but it allows you to cross gender lines without fear of judgement
For a woman to have her strap on sucked it provides an opportunity to cross gender lines. You get an experience you never get. You get to have a penis.
As for a female wanting to suck a females strap on I cant see any reason why you would want to, but I am sure one exists.