I'd have to read it again but I think it said he was so angry at the sight of this he struck/slew him(Onan) dead immediately,she(Tamar) dressed as a prostitute to trick her father in law into having sex with her since the promise was not kept of a marrige to Shelah his brother

Did they have a choice not to mary her, it was the law ?

I read Juda was the one offering Onans brother Shelah to mary her ?
And was the one who did not let him,indicating to me he really had no choice,and why God praised Tamar
for tricking her father in law...since I quess God thought he should have....I have to study this more as well

Why so many until recently practicing this in America ?

had to edit after a re-cap on this

still sick and bizarre of God in the bible(IMO) " To Me" it appears they had no choice ????

What happen to freewill here ???