Ok let me help you out a bit in your approach.
Your questions are valid but only valid if you are dealing with a logical individual.
Your mother is not logical, she may well not even be completely sane. She threw you out of your house at 17 because you had birth control pills. First, I'm not even sure thats completely legal, you were a minor, second its most likely the dumbest thing you could do to someone who you are worried about 'going down the wrong path'.
Its got a lovely cruelty that would fit in well with the old testament but thats as far as I'd go making it any way holy.
This episode tells me you are not dealing with someone with a full deck. If you wanted to have fun with her, blame your swinging on that year she kicked you out and how you had to do 'things' to survive. Tell her you hope god forgives her for abandoning her child to the sodomites.
Seriously, I'm sorry but what she did was fucked up.
So she spied and you and found out you were a swinger.
Well over all your questions are going to be meaningless. For someone who would kick out a 17 year old girl to the street, no logical argument, or scientific study on STD's is going to matter.
Good studies on swingers are rare because most good swingers don't participate in studies. While my feeling is swingers are on par to maybe even lower on the STD scale, you will be hard pressed to find real data. No one puts 'swinger' on their medical history if they care about being outed.
You may have to end up cutting off your mother on this one, and even with the little information you gave us I think it might be a good thing.