Wife admits she loves us “differently”

My wife and her bf have been together a bit over a year now. But they’ve known each other much longer.

Last weekend as he was leaving, and they were saying their goodbyes, I heard her tell him that she loved him. It didn’t seem like anything new to him. He said replied in kind and they kissed, hugged and he left.

I didn’t say anything at first as it caught me off guard, I guess.
When I did bring it up, she was surprised that I didn’t know this already.
She told me that she loved us both for different reasons. Dean, the bf for more physical and “masculine” reasons (something she said was difficult to explain). She loved me for our solid foundation and our history.

Last night she told me that Dean and her talked about this and he wants to sit down with us and talk this out and define our situation better.

Sorry. I’m not much of a writer. I’m mainly looking for advice maybe and to get the views of others in this situation.

How common is it that the wife falls in love with her bull or bf?

Thanks in advance