Female suddenly aggressive towards her husbun - opinions/advice needed

This afternoon was the first time I’ve seen this behaviour between my buns. Petunia, the white bun (f, 3.5 months, scheduled to be spayed soon) started mounting Gilroy my brown bun (m, 2 yrs, neutered).

Obviously Gilroy was offended by this and tried to put her in her place but she appears to not get the hint! This happened several more times outside and then inside… each time I broke it up and separated them. They’re normally free range throughout the house and backyard.

I currently have them inside, separated for the night so I can sleep in peace but I’m worried about how to proceed if this continues tomorrow. Any insights, advice please. 🙏🏻