Ask your doctor, and you'll probably hear the advice to wait with sex until your six-week postpartum appointment is out the way and you got the green light. Only a rare parent would find the time, energy, and will to have sex before that, but they do exist. "Help! We want to have sex, but is it really OK right now?" a friend texted me three days after she gave birth, during her first night home from the hospital.
The answer to her question is generally that it's not wise. If you tore during birth or had an episiotomy (vaginal surgical cut), infection is a concern. Your pelvic area will also still be really sore, and sex is likely to be unpleasant at this point. Many also prefer to wait until lochia or postpartum bleeding has stopped until they get it on. As for the "will it feel different" bit, I can tell you from experience that that's very likely.
Your vaginal wall will be more relaxed and wider after giving birth (unless you had a c-section), and it may hurt a little the first time. My husband said it was different, but not less nice. I don't know whether he lied :). This is not forever though your vagina will return to normal, or slip into a new normal. At the risk of giving too much information, I'll say that mine is probably the same in tightness after two kids, but it looks different. I tore, and the tear healed in a way that left its mark. It's just one of those things.
I have stretch marks too. Yes, they fade, but you can still see them. And you know what else? I have grey hairs, and wrinkles on my face too. Well, I am in my thirties. Sex is still good :). Postpartum sex is best when you take it slowly and don't expect too much, too soon. Listen to your body and stop if you feel discomfort. It will get better. I promise. And if you're lucky, it will be great from the start.