Homosexuality is not a sin in Judaism, for the simple reason that it is not acknowledge
d anywhere in Jewish texts as the social and psychological phenomenon
modern society knows it to be.
What makes practical homosexuality impossible for men are the above-mentioned prohibition on spilling the seed, as well as the age long taboo on sodomy.
As for women, well, it's not strictly speaking a mortal sin (there's no mortal sin in Judaism, kids, I'm being annoyingly ironic) for two women to snog each other, but there are mentions (in texts later than the OT) of the undesirability of oral contact with the vagina, or mutual masturbation. Since It's Not In The Bible, however, I suppose you could get around that one.
As for the original question about marital purity laws - why would two women observe them, anyway? They only apply to male/female contact - there's no problem with women touching women during menstruation, after all. Come to think about it, there's no problem with women touching men, either - it's the other way around. I'd say Deborah909's friends were being overzealous.