Well all guys are different, so their individual motivations are going to differ. In general, male mating strategies do differ from female strategies on a biological level. How that plays out can be different, though.
The goal of an organism is to pass its genetic material on if it can. Because men can have an opportunity to procreate pretty much at any time, it is an advantageous procreation strategy to mate with multiple women. Women on the other hand, are only fertile at certain times, and when pregnant, are more vulnerable. An advantageous strategy to ensure their genetic material gets passed on is in selecting a mate with resources or an ability/desire to protect the mother until the offspring are capable of surviving on their own.
Now even though most men don't actually want sex to result in pregnancy these days, the drive is still deep in the wiring. Also, because we are more evolved than we once were, we are able to see that ensuring our genes are passed on can be more evolved too. Because children require a lot of nurturing to turn into healthy adults, It can be ideal for the father to stay engaged so that it is likely his genetic material will be passed on to his children's children. After all it wouldn't be good to have 10 kids, but no grandkids (from a biological perspective).
As far as it plays out with respect and no longer being into you, that is complicated. When we are enamored, the chemicals make us think in certain ways that we do not when the subside. So it is fair to say that we may fully be more engaged before sex, and when the hormones subside, we feel less connected. Of course oxytocin is released after sex, and that encourages bonding, so that can affect things too. Our bodies dope us to make us want to have sex, and then those chemicals fade.