low libido

I'm 20 and it seems like I'm never in the mood to have sex with my SO anymore. He always wants it and he tries everything to turn me on and it rarely ever works. I still love him deeply but I don't know the reason why it's gotten like this. I don't know if it's my birth control or the fact he's gained a lot of weight even though it doesn't bother me. Is there anyone else out there that rarely has sex with their SO anymore?