It was because she got dirty in the barn. There was a cheesy scene where the vet asked Meredith to go to his house and he'll cook dinner and then they started talking about sleeping together. Meredith said she wouldn't sleep with him, and the vet said, you have two choices, "1. You go upstairs, get naked, take a shower, put on one of my shirts and we'll have dinner. 2. you go home." Meredith said " I'm not having sex with you", and the vet said, "If you take a shower, I will not have sex with you."
So, McDreamy came in seeing her in his tshirt and thought they had sex.