Hi Jay,
Looking through some of your other posts, you appear to be a pretty smart guy, so you likely know this already...
We (males) peak sexually at around age 18, so it's (albeit gradually) all downhill from there.
We tend not to notice much difference during our 20s, but around 30
ish, we begin to notice things like this (and muscle changes, weight control, others) more than we did during the previous decade. Of course some of this has to do with genetics, over which we have
no control.
What kind of stress are we talking about, and what kind of weight gain? Both are going to affect sexual function/response to some extent.
Best advice I can suggest (aside from seeing your doctor) is what you suggested to someone else in another forum -- watch your diet and get some exercise (both of those affect stress & weight).
Doc