If you have a problem with hormonal birth control (as I did) you can also ask about copper IUDs, which aren't hormonal. However, hormonal IUDs (such as Mirena) are typically much less harsh on you than other hormonal birth control. Still, I thought I'd point it out (especially since depending on the model, they last up to ten years!).
I've been wearing a copper IUD for three years now and all the problems I had due to hormones vanished pretty much right away. It hurt to have it put in, but then it just felt like menstrual cramps for a couple of days and I was fine afterwards.
Each body responds differently, but personally my periods have been less painful since and I stopped having migraines as well. Also, while I had more blood loss the first few months, I know have much, much less than I ever did on the pill.
If you opt for a hormonal IUD, periods are much smaller and might even vanish completely, although it varies from one woman to the next. I hear copper ones typically increase the flow but it's not always the case (wasn't for me).
One thing with IUDs it that if you do get a STD or STI, having a IUD in place can complicate it. My solution? Don't catch one! But it's something to keep in mind. It also only prevents pregnancy in your uterus, meaning it's not going to stop you from getting one outside the womb (although it doesn't cause them either, and they're rare, so I don't personally worry about it. But it's true that most other forms of birth control work for ALL pregnancies, and this one is more focused. Obviously a pregnancy outside the womb needs to be terminated anyways for health reasons, but it's something to keep in mind.)
I personally love my IUD. I think the best form of birth control is the one that works for you, so I tried to be fair and give the pros and the cons. Feel free to read up on other methods too and/or ask your doctor so you can make an informed decision.
Going back to the subject, I really care about fluid bonding. I do believe it's a matter of trust, mostly trusting everyone to at least use condoms or other protection if they screw around, and to a lesser extent trusting them to to screw around. But I'd feel more comfortable with someone I know is likely to screw around but sure to use protection than someone less likely to screw around but who wouldn't use protection if he did.
Regular testing is also a good plan. If someone has something for whatever reason, you want all the others to know.