Don't do it! If she loves you and you love your m3. She has to understand! Or you could compromise by lessening your hornyness!
Seriously - There are a few models I long for EVO's, STI's, GTR and M3s
Mine - Evo X Hers - E350, after a career upgrade she had to get a gas saver (ironic I know but her job requires heavy traveling) So she ask for me to change my car. I stupidly agreed. Comes the CRZ, a hybrid it's good... I get to drive the Benz more but no. The car is a truck and its no EVO. I begged for an Evo but she wants a Benz to keep. We went to MB dealer and ended up with a c63. I got suckered by the exhaust note and I was happy for a few months. Finally I put my foot down and when we could do it I opted for my 2013 M3, I'm just glad she also said yes being a luxury brand. It cost us a lot in the long run. But now I'm back to my happy state with my M3. Definitely way better than the EVO by I would have been as happy with the damn thing. So, please don't do it or you will find yourself in a whirlwind situation.