Is it improper to use the groom's parents house for a reception?

My fiance and I are wanting a small reception (80-100 guests) and would like it outdoors. We were hoping to pitch a big, white tent and set up tables and chairs. His parents were thrilled with this idea and offered up their backyard to use as the reception. We love the idea b/c it will cut costs! Who wouldn't like that? Now for the kicker, my parents are big on etiquette about who pays for what, blah blah blah and they despise this idea and think it's wrong for us to do that. I never asked them for help on the wedding and my fiance and I are planning on covering most of it. It has been WWIII in my household and my parents think someone else is trying to plan the wedding (mind you this is EXACTLY what we want) and my father thinks this is my day and my mother's. Why is it so wrong to have my fiance's input??!?! This, afterall, is OUR day...nobody else's. I understand that they want to help, but the whole issue is they will not be COMFORTABLE having it at someone's house. It won't be inside, it will be in a huge backyard, so what's the deal. Am I wrong for doing this? Would this offend your parents (brides) if you decided to have a backyard reception in the groom's parents' backyard??? I don't know how to go about putting my foot down and saying "too bad, this is OUR day and suck it up" without hurting their feelings. I love my family and I want EVERYONE to have a good time!!! It's a celebration of love and committment! It doesn't matter where!!! I would understand if I expected them to pay for a huge wedding in some outdoor botanical gardens, but this is free!!!! I'm lost. I feel like I'm making this everyone else's day instead of a day for myself and fiance to celebrate with others. Anyone else out there run into the problem with your parents being upset using a free "venue?" I'm so upset and stressed. This is supposed to be fun!!