Have you considered just seeing your lover one night per week? Two nights is a lot for a relationship that's basically only sex when you have a full-time bf of your own. This could be a perfect compromise in that you still get to enjoy regular time with your lover, and your bf gets to see that you're willing to make a major concession (50% less time with the lover) for his sake. But in return he has to stop asking these invasive questions about your sex with him.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.