Why would it be? Penetration? Women can do that with fingers, toys, tongue (as Ivy likes to show off). Hell, a woman could fall on a pointy stick and be penetrated, doesn't mean she had sex.
I think it has to be two (or more) people intentionally and consensually doing sexual acts to each other. After that it's just a list of techniques to tick off a list.
But, the heteronormative definition (even legally in some places) does say penis in vagina, which is a narrow definition.
In the game it's pointed out (sometimes) as equal to Ian hooking up with someone else, like when you had Alison or Cherry over the night before, Lena thinks to herself that she too has been fooling around on the side with Louise (if you opened that path). It shows off the double standard.
Now, historically there was kind of a reason to ignore WLW sex, because to society it was all about children, and a woman couldn't cuck you with a baby that wasn't yours. So it was a blind spot (at least at times).
Often, when discussing threesomes the way the letters are arranged suggests how the participants interact. Like Ian/Jeremy/Alison would be MFM, because both men interact with the woman, but not each other. If they did, it would be MMF. Same goes for two women and a man. The Lena/Louise/Pick Guy threesome is a FFM, but not a FMF, because they interact directly with each other, not just with the man.