I believe we should strive to keep every commandment we believe is "valid" - every commandment that we feel can be attributed to God / inspiration / revelation. I just think we have to be VERY careful never to equate that with every commandment we like naturally. Without commandments that require we do what we don't want to do naturally, we would experience no growth and change. To me, the concept of an Atonement / grace that bridges the gap between what we want to do / become and what we actually are able to do / become is the key - as long as it doesn't rob us of the motivation to change. In many ways, commandments and constraints are what we make of them - but I don't believe in constraints just for the sake of constraint. I also don't believe everything that many people frame as commands actually originated from God as universal command - and that's an incredibly important principle for me.
A good example of this is sexual activity. It is powerful - both sublime and terrible - binding and destructive. It is what we make of it.
I have no problem with some general constraints on such activity within the Church and the Gospel (no problem with the principle of a Law of Chastity), particularly in our current society that leans toward no restrictions and, as a result, is full of really, really, really, really messed up people and destroyed relationships. On the other hand, I am saddened by the general insistence on treating discussions of sexual activity as taboo - on promulgating a prudish, Victorian attitude that is nearly as destructive as promiscuity. I also am discouraged by our general insistence on simplifying such a complicated, far-reaching issue into a binary, black-and-white view that misses so often the complexity of the scientific reality. Human sexuality is something we still understand only partially, and, while I believe our theology has the power to encompass so much that we still don't understand fully, I believe we haven't touched the surface of that power in many cases relative to this topic.
At the most basic level, I think we simply have to remember that we are not a collection of homogenous individuals - that the best standards for communities and societies are not the same as the best standards for some individuals (especially those who are "different" in some way) - that communal standards tend to center around acceptable minimums for the majority (and often radically misunderstand multiple minorities) - that what is not destructive for some persons often is destructive for other persons and/or the collective people as a whole. It cuts both ways, and, unfortunately, it's really hard to implement commands and constraints throughout societies that don't end up cutting portions of those societies in real, painful, "unfair" ways. It's important to remember that, even in cases where it doesn't change anything in practical terms.
People are complicated, and we do each other a grave disservice, in my opinion, whenever we try to categorize everyone in a strict binary system where only two options exist - or, in many cases, only one. We must have communal constraints, but we also must have the ability in most situations to accept individual exceptions to those constraints. After all, we as Mormons have a long history of wanting others to allow us to be a peculiar people; we can't expect that, however, if we don't allow all (wo)men everywhere the same privilege - if we end up exercising unrighteous dominion in our zeal to promote the commands and constraints we accept for ourselves.