Wow. The "duty" word really gets people going...
I came to this idea on my own, long before I got back together with Ares. It is a personal belief of mine, that meshes pretty well with my faith and my family. It starts out logically. What do couples (triples, polycules, etc) fight most about? Sex and money are at the top of the list. So, to ensure a happier relationship, it helps if the partners are financially responsible, sexually talented, and willing. I hear so many people complaining about how their partner is either bad in bed, unwilling to have sex, or whatever. I refuse to set myself up for failure in that area.
Obviously, if I've got the flu or something medical or psychological is really wrong, I'm perfectly fine opting out because I've got a good excuse. Other than that, I do it because I enjoy it and also because I love my partners and it is probably the very best way to bond.
Here's where things get counter-culture

. My belief is a rejection of the "me first" autonomy that is part of modern American culture. It is an acknowledgement that because I live in a group relationship, my needs and wants don't always come first. When I joined my faith, I learned that my faith supported the beliefs I already had. I refer to 1 Corinthians, Chapter 7...
"3. The husband should give to his wife her conjugal rights, and likewise the wife to her husband. 4. For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does. 5 Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer; but then come together again, so that Satan may not tempt you because of your lack of self-control."
In other words, since I am in a relationship, I have agreed that I don't have exclusive 100% say over my body. My husband has the same duty to me. The concept of the relationship we have involves both minor daily sacrifice as well as the radical willingness to make major sacrifice. My husband demonstrated that willingness many years ago when he saved me from a man who was going to rape and murder me. He took a bullet in my place without hesitation. He also later helped me overcome an addiction, and remained my friend for years after our first relationship ended. I owe that kind of loyalty and willingness in return. By extension, I believe that Swift deserves the attention and physical interaction she may need to feel content and cared for - I find it disheartening that I may have done less than my best toward her.
I will admit, I've definitely underestimated how tired pregnancy would make me. I feel more than a bit overextended, but that's my situation as a whole and not any one thing. I suppose I could lessen my load and rely on my partners more, but my reluctance to do so is more due to stubbornness and pride than anything else.... I used to be able to have sex several times a day quite happily, and I dislike my new lower energy level and have yet to adapt to it. I'm envious of how easily Reina and Swift seem to go through pregnancy...so little sickness, discomfort, and plenty of energy. I look and feel like I got smacked by a truck.
If it seems like I'm giving up a lot, I feel that what I get in return is so much greater. There's practical stuff - I work full time...when I have my child this fall, I can continue working if that's what I want to do. I know that loving care is guaranteed, and that's a big job! Dinner is on the table, the house is kept, the laundry is done, and I haven't been asked to tend to any of it. There's emotional support - pregnancy has scared me more than anything in my life. But I live with two girls who've done the mommy thing for a while now...that's immensely reassuring. When I have to see the doctor and I'm freaking out, one or both of them has been there to hold my hand. I don't just get cuddled by one person at night or when I need it, I get cuddled up in a pile. And when I want sex (my drive before pregnancy was pretty high) I know I'll never have to work for it or plead for someone to participate...that need will be taken care of immediately and I will be getting some of the best of my life.