It's very pity, when something awkward happens in bed...
Experience doesn't deliver men from fear of sexual failure. When a man grows older, it doesn't mean that he can't fail at all. More with that, when he works a lot and has no time for his family, he is able to fail much more often, than when he's young and has no worries at all. And it's very hard to find out what's going on with your man when he declines on sex, cause men are not those who talk a lot about their failures. When something awkward happens, he would rather prefer to keep it inside, but that's no good and that leads to more failures in future. So even if you're young and sexy lady, you should know what to do when man is bad in your bed. Even without talking and sorting it out you can make things better - the only thing you need is imagination and love.
If your man is still kind and tender to you, but he doesn't want to have sex at all, maybe he's afraid of something. That happens lot more often than lovers. So don't be quick to judge your beloved - try to do something for him that would help him feel better and more confident.
Confidence is really important. When a man is afraid of failing, he is most likely to fail, because he programs himself with these thoughts and result will be sad. So when a man wants you and wants sex but he can't do it because his penis isn't hard enough, you should help him. Viagra will do, but you can't have generics near all the time, and they affect health after all. So the best way is not hurrying, not judging your man or screaming at him. Start with a blowjob - be sure, it will work and in the next five minutes a gentleman will be fully ready to give you the sex of your dreams. Also you can masturbate for your beloved - this often helps a lot, especially when you're not performing this too often. But be careful with masturbation - sometimes a man prefers his hand to a woman because he just got used to it. Don't lead to that - it would be a mistake.
And the second moment that can really be sad - when one of the partners gets his orgasm sooner than the other. This is a problem of most couples, so if that happened, don't think that you're unlucky. Doesn't matter who was first - he won't be the one to help his partner, so it's better to make sex longer. A condom will help or pauses in sex - just choose something that is quite helpful for you and you will see how you can make love all night. Don't hurry - sex brings pleasure when it's slow.