For a slightly different angle:
http://yesmeansyesblog.wordpress.com/2010/01/18/i-cant-say-no/

Sure as hell beats the tried-and-tested “Not tonight, I’m reading about molluscs!”

When you’re in a relationship, it isn’t all about you. But it isn’t all about the other person, either. When you privilege either person’s desire for sex over the other’s just not being in the mood, you create a very unhealthy dynamic that takes away the right of either person to say no.

Obviously you have to balance each other’s needs when they conflict. But at the end of the day, I’d rather have a relationship where sometimes I don’t have sex rather than feel entitled to something, no matter what, even when my partner isn’t into it.

on the other side of it. the person who is interested in sex needs to be aware that sometimes people will find themselves uninterested in sex for reasons that have nothing to do with their partner, and not to take things personally when someone is not feeling well, or is mentally, emotionally, or physically tired or distracted.

it’s a two way street of understanding.

As of late, if he sees the ice pack on my head, he KNOWS a raging migraine (REAL, I get them very very badly) is in progress and will actually suggest a back or neck rub instead of sex. (By the time I get to the ice pack and painkiller stage, it is too late for an orgasm to halt the headache in progress.)

Only lately does he on occasion turn me down. I’m a woman, I’m not in my 20s (eh ehm) anymore and my drive is actually getting stronger, he’s getting older and although his drive is still VERY strong, he sometimes doesn’t “feel like it.” WTF? I am not used to that. If he’s funny about, “I don’t want to play Sex right now, I’m really tired, OK?” “No, No, Nympho Queen, I cannot take any more of your torture right now!” is OK. But just a while ago, we’d gone about 2 1/2 whole days, he actually nearly yelled “NO” when all I did was put my hand on his leg, and he was saying NOTHING, while I was hinting (ug, he’s a man.)

It finally came to tears {me) and raised voices {him) when I confronted him with “ignoring” me and “taking advantage” of me, as just a few days before we were at it like rabbits as usual, and now I was upset because I felt ignored (and horny) and he was upset BECAUSE I was crying (as per usual, in man/woman relationships.)

Turns out he was having a nasty case of what was probably Jock Itch, and he was in actual pain. WHY didn’t he SAY something? I have NO idea. (Why didn’t he do what I do when I have a yeast infection and take care of HIM with my panties on, as to reinforce “none for me, thanks” ? No idea, either. This is new for us. We are going to have to learn how to work with this stuff.)

It’s really better if we both communicate. As he gets older, I fear this may happen more often (I don’t see my drive as going anywhere South any time soon.)

Men don’t pick up on “signals” or “clues” well, like “ice pack” means I really really have a headache which sex isn’t going to cure, or I’d be right there, or heating pad and my back turned in bed means “Cramp Emergency, back turned in bed means “not the kind of cramps that sex would help. You missed that window by about 6 hours.”)

So, as women we have to do two (maybe 3) things. 1)Let him know, why you don’t want to(Not with a LOT of explanation, guys hate that. The facts and nothing but the facts, ma’am.) and 2) NEVER EVER fake a headache or cramps, if you don’t have them. Dishonestly will get you nowhere. If you “just don’t want to” offering a blow job, or a hand job, or just watching him and giving moral support, while he is taking care of himself is always an option. and 2) (Do I have to say it?) Sex should NEVER be used as a reward or a punishment or as part of a scheme to get what you want or to “teach him.” Also, investigate your “just not feeling like it” usually if you get started, you’ll enjoy yourself.

It’s amazing how much people can HAVE sex and how little they TALK about it. It’s really important to talk about it, often.

Try being a guy and telling a woman it’s not happening tonight.

Guys get a bad rap sometimes for being sexually aggressive, pushy, not respecting womens’ boundaries… but have you ever seen how a woman gets when she’s denied sex!?!? They’re HORRIBLE! They’ll pout, they’ll rub other guys in your face, they’ll call you gay, they’ll threaten to cheat… Awful!