Dear 100 Hour Board,
How close of blood relation is too close for your to consider in marriage?
Assuming you have the relationship before the family tree stuff is figured out
Going with the assumption you laid out that the relationship was already in place before the familial relation was discovered, I'm gonna say 3rd cousins would be too close. I have a lot of 4th cousins (most of whom I don't know) because my great-great-great-grandfather had several wives and many children (you know, back when polygamy was sanctioned by the Church), and I don't consider them to really be family aside from an occasional joking "Hey there, cousin!"
Like Zedability says below, if someone was introduced to me as a cousin it would be weird. However, if I met someone and started to get to know them and then found out they were somehow distantly related to me, I don't think that would be a deal breaker. I think 4th cousins might be too close in that case, but I'm not sure I would rule out 5th cousins.
I'm going with Liz Lemon here:
Grey: "I think we're third cousins."
Liz: "Yeah. I'll see you at the reunion."
As stated in a prior question, Andy and I are 9th cousins (removed some number of times).
One of my best friends is my fourth cousin, so that feels too close to me. Our mothers have the same maiden name, which to me denotes too high a degree of relation.
If I was already in a relationship and discovered we were anything closer than fifth cousins, I think that would be a deal-breaker.
Marrying your second cousin is legal in all 50 states, so. . . .
For me it would be too close if we had any chance of running into each other at an overextended family reunion. With my mom's side of the family there are semi-frequent gatherings with all of my aunts and uncles and cousins, but also my mom's cousins and all of their kids, and sometimes even more random people who I'm not sure what our relation is. Even if we hadn't ever gone to the same reunion, but if we had both at least heard of the same one, it would probably still be too close of a relation for me.
Somewhere in the range of 3rd-5th cousins, but it would depend a lot on the circumstances. If I meet someone in the context of "This is your nth cousin!", that person is off the table, no matter how distantly related. That's just weird. On the other hand, if I were dating someone for a while and found out we were distantly related, there aren't really any genetic drawbacks to marrying a 3rd cousin, and any worry about it is just the result of social taboos that are relatively recent.
In Alberta, it probably would have been impossible for me to not be made aware pretty quickly that someone was my 3rd cousin though, so it would have ended up being 4th or 5th.
At the same time, part of the reason I chose BYU was to get away from the relatively tight-knit social culture in Alberta. I wanted to be able to date people without being told whether their great-grandpa was a good member of the Church or not. So that pretty much guaranteed that this wouldn't be an issue for me, and it wasn't.