I have posted on this topic before, but I have had a request for more on this subject. As one goes through this kind of a protocol to addressing their IC, they will find that the vaginal tissue will respond along with the bladder sensations. Typically in the beginning of treatment, the vagina is often as painful as the the bladder, making sexual intercourse nearly if not completely impossible. Commonly, the vaginal tissue is dry, but, it may be simply painful and not dry. Orgasm can cause more pain, and, to make matters worse, the aggravation caused by sex may last for days. This is also true for males having IC. Sex is a huge part of what complicates the life of someone having IC. However, the flip side is that, with time, sexuality can be normal again without pain! Finding one's way to that is the difficult part because of the many factors involved. First and formost, the emotional affects are difficult to address. This is because even as the pain starts to abate, improvment is difficult to trust. Being able to once again start engaging in one's sexuality, often a part of the person having IC that has been necessarily turned off for survival is very involved. Some people do not disengage and have sex regardless of the pain while others shut off that part of their life. How people handle this really varies. However, for those working through this slowly and gradually, the question is how do they know when to go through it and when to back off. I think the very most important thing to impart is that number one, sexual fluids (sperm mostly) should not be communicated between people. So, either a condom should be used, or in many cases, withdrawal method is also acceptable. If the vaginal tissue is dry, a lubricant should be used. Jojoba oil is often great. Using many of the OTC products subjects a person's body to all kinds of toxic chemicals that can exacerbate pain and increase one's exposure to carcinogenic chemicals. Taking sex slowly and gently is very important. Allowing both people to enjoy the touching and the pleasure without increasing tissue and swelling ore than necessary. For women, there have been studies that argue the benefits of  sexual intercourse because activity increases estrogen in the vaginal tissues. One's own estrogen is healing, and therefore a benefit to someone having chronic inflammation in the area. I think the "trick" is figuring out for one's self how negatively they feel they are impacted by intercourse and for how long to access how often they feel comfortable engaging in it. To help in minimizing symptoms, washing with gentle soap and water afterwards is a good idea and for women, using some jojoba oil over the vaginal tissue helps sooth aggitated tissue and will minimize the after effects of sex. Men can do this as well, but I am uncertain it will have the same soothing effects that is does for women. For men, if your female partner is involved in your condition, using a condom is critical for the success of your improvement. As your treatment continues and improvement in your health becomes greater from within your body, changes will occur in your external tissues including the genitalia. Orgasms will eventually not create more pain (for those that experience that symptoms. not al people do). 
Many of my patients have been of the undestandable opinion that their sexuality will never return to them, and in almost all of those cases, it has returned. Folowing your particular protocol, food, supplements, etc.. is as important to the improvement of your sexuality as it is your IC. Trying to maintain your romantic life is challenging when you have IC. So, again, the basics are: do not share sexual fluids (the exchange of organisms through sexual fluid is big with IC), wash after sex and then apply something soothing like jojoba oil, and experience the sex slowly and gently and not more times per week or month than feels comfortable to you. Learning to iisten to your body is important with food, activity, and sexuality. If something is causing tremendous pain, then, I would say, back off a bit. This means it is too much. Wait a week and try again. Work with your body to get a "read" on what it can handle. Slwoly, you will come to trust what it is saying. Your body IS talking to you. Learning how to hear it, is the trick. This is somthing that each of you will get better at with time. And, it will serve you your whole life. But, more than anything know that trust comes with change. When you see change occuring, you will trust that. And, nothing anyone says will change your feelings about something. You must experience those changes to trust them. So, time is the huge element that is involved with change. As time passes and you experiences change, trust will come. On this road, there will be many changes in your emotional and physical feelings. How you experience them and learn from them will make your journey less or more enjoyable. So, go easy on yourself, be understanding and loving to your body, and...listen.