Why would a guy ask if I've ever had a boyfriend?
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Maybe he's building a psychological profile of you. That's how you would do it: getting to know the boyfriend.
Maybe he wants to know how your past relationships went and why they failed so you two could avoid them in the future or it gives a leeway into talking about dealbreakers.
Honestly, we don't know. All we can do is guess. It's better to ask him about it and see what he says.