Has anybody ever heard of having to divorce, for security purposes, when consindering joining the Navy SEALs?

Whoever told you that is yanking your chain and is full of crap. To put it bluntly. The military does not care whether you are married or not, no matter your mos, job, specialty or position. And certainly not as far as a security clearance goes. Now you being married and seeing anything above a secret makes it more intensive as they have to look at not just you but at your wife as well.

But short answer no, the SEALs do not and never have mandated for security purposes that a serving member must get a divorce. That is somebody not knowing what they are talking about or flat out lying to you.

I don't know where you got that idea from. The only thing that comes in to play is gaining a Top Secret Clearance; the military won't make you get divorced but they will ask you about contacts overseas

No. Unless of course you are talking about a potential SEAL recruit being married to someone who is from or has ties to a country of concern or terrorist organization. This could hinder acquisition of a TS clearance. However, just being married has no bearing whatsoever on obtaining any military job, SOF or not.