In regards to the decline in momentum of sexual exploration: when first meeting each other our games are at their peak levels and we are giving our all in order to get down into those dark areas below. Once we get there we still need to keep on top of our game in order to impress the other and show them how talented we are in the ways of sensual pleasures. Different positions different areas of the room or house, outdoor areas, public areas, underwater... whatever. None-to-soon however, we become accustomed to the other. We learn how to get each other off with minimal effort and then that’s all it takes; all that is needed. No more theatrics no more acrobatics, just the flip flop, bang bang-splooge. People say sex is like pizza even when it is bad its still pretty good. How to make bad pizza better? Dress it up! Throw some fucking pepperoni on there with some sausage and meatballs, peppers, onions and mushrooms olives...etc. Sex is no different. Dress it up. Throw some tools in there. Roll playing, exhibitionism, maybe some S&M... also get back to the raw impulsive ‘I’m gonna fuck you where you stand’ sex and forget about preparations and proper clothing removal. Throw her/him over the nearest object and have at it until you hit the floor!
Long term effects; building boundaries and limitations for your partner without knowing:
I don’t think that the age of couples necessarily has to do with the 'sex no sex' factor. It’s moving in with each other that decreases the appetite for sex in one or the other or both. It becomes more mechanical than impulsive and you need to put extra energy into making it creative which generally you don’t want to do because you already been inside that/on top of that countless times and the lack of motivation often outweighs the desire. Rolling over and pumping one out in the morning becomes as thoughtless as the three S's that follow. No romance and no desire other than the concern of pumping one out becomes boring and then the mind wanders. When not living together there is the 'I miss you, I want to fuck you' factor. You still can have some fun making game tip toeing around trying to finesse your way into each others pants. But, once you get there you can’t stay out. And, if things are going well in the relationship you’ll get to the 'fuck first' period. This is when you meet up and screw several times before doing anything else. e.g. Its been a busy week you and your partner have a date to go out for dinner Thursday evening; get cleaned up/dressed up meet and instead fuck one to four times (amount of fucking and creativity generally correlates to how long you’ve been apart) then wash up and get something to eat probably something easy and quick rather than the nice restaurant you had planned. (Why go through the formal motions when the result has already been attained?) So be comfortable... relax... let your game slide, this person is into you. Finally things begin to plateau and a comfortable stable relationship is formed (and this is usually not just a 'plateau' but more often than not the actual 'peak' of the relationship). Now sooner or later, someone will eventually mention, for one reason or another, "hey we should move in together". This is the exact point of no return for terrible or worse. This is where the relationship becomes 'over the hill'. Yes, you can put off moving in with each other but only for so long. Sooner or later you will have to move in. If you do not, well, then the relationship is doomed because eventually the other will look elsewhere for someone willing to make the deeper or fuller commitment that is 'moving in together'. Once moved in there things become more routine and more gritty and human. You begin to see the person for what they really are in all of their sick ritualistic day to day life. Again, sex will be routine though less and less creative and passion even diminishes. Fuck work eat fuck sleep fuck or work eat fuck, fuck sleep… doesn’t matter. Comfortable relaxed accepting you take each other in strides and eventually for granted. Sex is no longer a goal but something to fit into your schedule. Eventually, a day comes along when you realize that you get more excitement out of your favorite porn site.