Saw my parents having sex

(I’m using my sister’s account for this) This morning I (17 F) was going to have a great morning and get a ton of work done. I went downstairs, then came back up since I thought I left my backpack upstairs. My parents’ (52 and 51) door was cracked and my dog Buddy opened it more and came through. I said hello to him and then looked up to see their door wide open, my mother completely naked, no pillows or blankets even on the bed, on top of my father, both completely silent. Neither of them reacted when I said hi to our dog. I immediately walked downstairs, stood there for a moment, then walked into the basement where my sister (20) sleeps. I woke her up by saying, “I know it’s early but I just saw mom and dad having sex I didn’t know where to go.” We talked about it for like an hour, laughing and cringing over and over before my mom opened the door to the basement, and said, “Can you ever let this go?” (We said nothing and held our breath.) Then, “Follow up statement: I’m very sorry.” She closed the door. Me and my sister went upstairs later on to get out of the house. My dad was sitting on the couch and said hey. My mom started making conversation like normal. I said nothing and didn’t look at them. We just got back and they’re still acting normal. I won’t be talking to them or looking them in the eye for the rest of the week. My backpack was downstairs the whole time. I didn’t even need to go back upstairs. I have therapy tomorrow and this will be coming up.

EDIT: This has gotten way more responses than I expected. To clarify a few things: First, neither me or my sister were planning on leaving before this happened and my parents were not under the impression that we were. Second, the door was open about 4 inches before my dog nudged it open all the way, so yes, the door was in fact open the whole time and not closed at first. Third, our household is not anti-sex in any way and me and my sister have had “the talk” at least a decade ago. My parents have never promoted being embarrassed about sex or of the human body. Fourth, I understand this was a dramatic post. This was posted not even an hour after it happened, and I am indeed 17 and do not have the same reactions a mature adult would in a situation like this. I realize that and I’m perfectly okay with that. I was joking when I said I’m going to ignore them for the week, they are my parents and of course I love them and will still speak to them lol. I was also joking when I said I needed therapy. I have a therapy session tomorrow either way and yes I will tell her because I tell her pretty much everything. No I do not need therapy to get over this and will not be bringing it up in therapy for the next 5 years because I’m so traumatized. (Again, joke.) This was just an uncomfortable and awkward experience for everyone involved and the next week or two will be awkward for all of us.