now I'm wondering if bi/straight women have a strong enough desire to have sex that it's worth it for them? Or if they fall in love with men and then just have sex to keep up the relationship? It's really unclear to me how much women like sex.

I'm a bisexual woman married to a man and I very much enjoy sex with him both because it's physically enjoyable and because it's intimacy. There's a very small element of "keeping up the relationship" but that's maybe like 1% of it. Just like continuing to go on dates and having deep conversations and sharing hobbies are parts of relationship maintenance.

You may be bisexual and homoromantic, meaning you desire sex with both but gravitate towards relationships with women. Bisexuality is a spectrum, some fall 50/50 while others are almost completely straight or almost completely gay.

You may want to explore this in therapy with someone who is familiar with LGBTQ issues.