i can see why someone would attribute this reaction (or at least parts of it) to patriarchy but i think this is not just a reaction men have towards woman-woman relationships but also many women have towards man-man relationships. thinking that it is less of a threat if your partner has sex with someone that is not of your gender, period. so maybe it is more of a heteronormative thing. but i also think it has a lot to do with not wanting to feel like you could be replaced.
for me, i try to work through these feelings by thinking about my own views on lovers. i only have sex with men and it's not like i'm on a quest to find the biggest penis out there and that's all that matters
every person i am with is unique and can never replace anyone else. person A might do something that i like more than when person B does it, but person B might have another trick up his sleeve that person A does not do. also, sex for me is so much about body chemistry. sure, you can learn moves and positions, but in the end what works with one person might not work with the next. since everyone is unique, the connection when two people come together will also be unique. some connections will work better than others, but that is not because one person is better than another. so that would be my advice, try to think about how you yourself view the different women you have sex with and see that it is not a competition. noone can replace you.