In the interest of not bringing readers down, but of staying true to the time period, I'll talk about my reasons for writing when I do.
Same-sex or three (or more) relationships happened. No matter what religious or country law said, they happened. People were careful about it, secretive in every aspect of their lives, but just because it was illegal and immoral doesn't mean it wasn't real.
Why do I enjoy writing m/f/f and m/m/f? Because everyone needs to find love and frankly my love isn't your love and all that...plus it's super sexy and ever so fun!
Today a same-sex affair wouldn't be looked at twice (in Western society that is, can't speak for other parts of the world). People may not like it for a variety of reasons but it is, quite literally, out in the open.
Threesomes...not so much. We're still a monogamous society no matter what we say. In that respect, my FBI lovers in Killer Valentine (and soon to be sequels) can't reveal their relationship to anyone because of the stigma attached. My sexy agents have to hide their love much like Sabine and Faith have to hide theirs from the Regency world.
The sad fact is that many people can't accpet. Pity.