SUBREDDIT: r/relationships
TITLE: My (17M) dad (53M) is pressuring my girlfriend and i to get her to go on the pill
POST: My girlfriend and i started dating a year and 4 months ago, 2 months ago we had sex and have had sex a few times since. Around 6 months before we first had sex my dad came to me and asked that i informed him when we did, at the time we thought this was a good idea because my dad seemed to be the only one of our parents that would react reasonably to this news and it would be good to have an adult that knows what is happening.

Skip forward to now, he sat me down and told me that she needs to be on the pill, (forgot to mention but we have used condoms every time we have had sex) this would be a very reasonable thing to happen, but for it to happen my girlfriend would need to tell her mum which she doesn't see as an option, as the mother is very uptight about the two of us having sex, i told this to my dad and he responded by saying that if my girlfriend didn't tell her mother he would.

Both me and my girlfriend feel this is incredibly inappropriate, this should be her decision to make when she feels ready, but ,e dad seems firm on this belief that this has to happen. What should i do?
TL;DR: 
My dad believes that my girlfriend needs to be on the pill despite us already using protection and is willing to go directly to her mother should she not tell her herself. What should i do?