The sex only happens in the 3rd degree, if at all, so is not in any way necessary for involvement. But it IS a big reason that Gardnerians have this restriction re: homosexuality and prefer to only initiate partnered heterosexual witches. It's really more of a result of Gardner's own prejudices and preferences, much like the inclusion of light bdsm and ritual nudity.. I have heard of no other established non-Gardnerian descendant tradition bar homosexuals.. QUITE THE OPPOSITE.
If having partnered heterosexual sex magic is the goal of the highest degree then same-gender relations don't really fit into their paradigm for ritual sex work. This is not necessarily homophobia, and I say this as a homosexual initiated in an Alexandrian coven where bisexuality was the norm. And mad **** was talked on Gardnerians for being so uptight. Also they are rarer then hens teeth these days unless you are in certain parts of England.
The Great Rite, where the high priest and priestess engage in sacred coitus while invoking the Lord and Lady of the rite being celebrated, is a very special sacred circumstance... rare enough I never saw one performed. If I remember correctly only 2nd degrees can attend anyways, which means working closely with said priest and priestess for a minimum of three years and three days.
So if you are encountering either pressure for sex or overt weird homophobia upon the initial encounter with any wiccan/pagan group RUN IMMEDIATELY FAR FAR AWAY. And then come here and give us the dish.. name names,. etc, as these are people who others should be warned away from.