Rapunzel, just before the story of Onan, Jacob and Esau have mention of wives(I know u already know)
I don't see nothing wrong with this,if it is what they "want" or others want,still to me it is not reason enough to take Tamar and have children if he did not "want" to, even if he could have took another wife. I only see mention of Judah having one after a quick reread though.

I am going to dig deep into the many wives condoning in the bible to understand it though now.

I'm sure it is appealing to "allot" of men that won't admit it

I don't see the reasoning behind it now of days though, but do during biblical times.