I think it's par for the course in any relationship to dramatically change the conversation any time former sexual partners get brought up. Sure, boyfriends and girlfriends alike are aware of the fact that there was someone before them, but I'd say most people like to live in the land of blissful ignorance and pretend otherwise. Thoughts like, "Oh sure, they've had sex with other people before, but never as good as me" is the ideal mindset. Otherwise you may end up tormented by, "Am I the best they've ever had? What if sex with their ex was.....BETTER?!" The latter is a slippery slope and will undoubtedly lead to an argument — or even worse, a breakup.
Stay in the now! Your relationship isn't about your partner's past, just like it isn't about yours. Avoid ex-talk! Unless of course you're being prompted by a major YouTube channel to spill your dirty laundry right all over your current lover. Then by all means, talk about sex with your ex — so the rest of us can sit and watch in horror over the most cringeworthy interactions of all time.
Awkward AF, amirite?! Speaking of sex with an ex, just don't do it! If you don't believe me, maybe you'll listen to science—
"A 2009 study in The Journal Personal Relationships found partners who stay broken up were more likely to report greater clarity in their lives than those who hook up with their exes. The researchers at Kansas State University noted couples in a cyclical relationship tended to be more impulsive about relationship shifts, like moving in together, buying a pet together, or having children together, compared to their counterparts."