Brain wondering during sex?

Hello all,

I’m happily married and have been together for 10 years.

We have a great sex life, which always has multiple orgasms and is fun / exciting. I’m still as in love with him and as attracted to him as I’ve always been.

Recently when we’ve been having sex my brain keeps thinking things like “how do you know you’re not a lesbian”. It’s a total turn off for me as it takes me out the zone. I’ve never been attracted to women and I’m certainly not now. But I don’t understand why my brain is doing this? Is it normal? Please help me work this out?