Is it Normal for a Straight Man to Obsess Over Men's Genitals?

Dr. Betty,
I have a kind of unusual question, and after seeing you on television, I figured you may be one of the best and knowledgeable people to ask. My ex-husband and I have a fairly good sex life. I think he is not as freaky as he claims to be. His idea of freaky is a threesome (with him running the show). But, since I'm not into it, we have not tried it. His hard-on seems to increase when he mentions me with another man. He frequently asks me about the size of my past lover's penis'.