…and a woman shall not stand before an animal to mate with it… (Leviticus 18:23)
This mitzvah is the prohibition against a female engaging in a sexual act with an animal, analogous to the prohibition on a male in the previous mitzvah. Remember how the zav and zavah were two separate mitzvos (Mitzvah #178 and Mitzvah #182)? That’s because, despite surface similarities (i.e., they both had a genital discharge), they were actually different phenomena. Here, too, the male and the female have distinct mitzvos. Despite an obvious similarity (they both involve having sex with an animal), there are inherent difference between what goes on with a man and what goes on with a woman. Since they’re actually doing two different things, it’s two separate mitzvos. (You can see that the Mishna specifically considered them two separate mitzvos on the first page of Talmud Kerisos.)
The reason for this mitzvah is as we stated in the previous one: God gave us the proper context in which to enjoy sex and this is definitely not it. He assigned roles to each of His creations and there are lines that are not intended to be crossed. If we can’t crossbreed animals or plow with different species, it should be pretty obvious that people are not supposed to have sex with them.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Sanhedrin on pages 54a-b and is codified in the Shulchan Aruch in Even Ha’Ezer 24. This prohibition is #349 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #118 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.