Just thought I'd chime in with some more contraceptive info. I've worked at a women's health clinic for several years, our clinic is non-profit and we have funding available so ALL methods of birth control--the pill, the patch, the shot, nuvaring, IUD's, condoms. etc--are FREE to women making up to 200% of the federal poverty level (for a single woman, this is about $1400/month). This is in OR, and I know a similar program exists in WA; not sure about other states, but I would encourage those concerned about the costs of birth control (someone mentioned paying $70/month for the nuvaring
) to call their local Planned Parenthood or similar organization to see what kind of assistance programs are available. No one should go without birth control because of the cost.
There has been some good advice on this thread, but also a tiny bit of misinformation. The IUD does not increase your risk of getting an STD, but if you contract one while you have an IUD in place you are at an increased risk of developing pelvic inflammatory disease (PID), a serious infection that occurs when untreated cervical infection--usually from chlamydia or gonorrhea--spreads further into the reproductive tract causing severe pain and infertility. So, the IUD is best for women at a low risk for contracting an STD--such as those in mutually monogamous relationships. The IUD is very safe and effective, and relatively simple to insert and remove (done on an outpatient basis by your OB/GYN in about half an hour, using local anesthetics), and there are hormonal and non-hormonal devices available, so it can be a great option for women who want a long-term, effective, non-hormonal birth control. We have seen a dramatic increase in the popularity of IUD's over the last few years, and many women seem very happy with them.
The ring is another good option for women concerned about hormones, it contains the lowest amount of any of the hormonal methods we have available currently. Most women experience few side effects on it, although some still do. It is very easy to use (ring is inserted vaginally once a month, where it stays in place and releases hormones directly to the cervix) and, like all hormonal methods when used correctly, very effective. I have been using the depo provera shot for almost ten years and am thinking about switching to the ring, in light of the new studies indicating long term use of depo can lead to loss of bone mass density. Many of the women at my work are using the ring and love it.
Good luck with finding a method that works for you! As far as the whole frequency of sex thing goes, only you and your partner can decide what is healthy for your relationship. However, this will involve some communication, which doesn't seem to have occurred yet. You need to talk with your husband about how he feels about the lack of sex, what you both want, and if there are underlying issues (personal or in your relationship) leading to a lack of physical intimacy. Be honest and non-judgemental with each other, and willing to compromise. I think in any long-term relationship, it takes effort to keep your sex life fun and engaging for both people. My BF and I have been together for three years, there are times when we are more or less sexually active but we make a point of being intimate on a regular basis even if we are both stressed, and to know each other's likes and dislikes so we are making an effort to please each other, not just mechanically "going through the motions."
Your relationship is worth the effort!
And yes I'm a female in a monogamous common law relationship. 
