Animal species engage in opposite-sex intercourse as a means of reproduction, or, as a means of sensual pleasure. Let's say that an animal possesses no knowledge that their activities with another of a different sex will result in pregnancy, and, thus, the continuation of that animal's bloodline. Therefore, they are doing it exclusively for sensual, hedonistic pleasure, and don't care about procreation whatsoever. Additionally, they don't do it with an animal of the same sex because, of course, they don't believe in pregnancy. They have no clue it exists. As there are animals who willingly refrain from sex, I would say it is due to political ideology, that such mating occurs. They objectively, the animal, consider such actions to be of a positive nature.

I don't understand what makes these women bisexual when they've chosen to identify as lesbians, live only with a woman/women, and gravitate toward sexual situations and media featuring women and lesbians.

Yes, I'm aware that romantic relationships should actually be fulfilling. That doesn't relate to what I've been saying.