It shouldn't. It diminishes over time but that happens married or not. No reason that a married couple shouldn't still be sexually active even as pensioners (try not to picture a mental image of that)
The trouble is, going slightly off topic here, that society and the media is obsessed with the amount and type of sex people have. This then leads to some kind of anxiety that your sex life isn't like what is described in these magazines. As long as you and the other half are happy with your sex life it shouldn't matter, but that doesn't sell magazines. Oh and 'mens*' magazines are just as bad. I should set up a company offering psychology for this 21st century problem, I'd be minted.
* I use that term lightly. I mean Men's Fitness and ilk.
I was kidding btw