I had a sim with the Soulmate aspiration and got the goal "Get married to BFF". By the time I got the goal, I was in a relationship with Soulmate status. We got Engaged through the conversation option...
There was somewhere I had seen this issue as well (still looking for it again) and apparently this is a bug. It was not intended to only have 1 BFF. It's very disappointing that EA can't fix these issues but instead desires to push out new (semi-broken) content or content that breaks things. o_0
Also, be aware that if your BFF dies, that's it. You can't get another BFF... again, #FixIt