In an article praising a possible long-term contraceptive injection, Cath Elliott manages to belittle the men who would take it by suggesting any woman "in her right mind" would use an additional method of contraception herself. The jab itself may be a dead end – trials report poor reliability compared to the female pill, and occasional long-term fertility problems – but the facts aren't really a contested issue. One irritation is the instant assumption that men simply can't be trusted to control their fertility.

Elliott's article might be leavened with humour, but the underlying tone suggests she also thinks we've been getting away with too much, for too long: "What everyone's really hoping for with this latest breakthrough is that men will finally start taking some responsibility for themselves, and about time too."

In truth, I have some sympathy. There are a few irresponsible men around – but the key word there is "few". In the arena of sex and contraception – the battleground that spills over into rows about deadbeat dads and feckless single mothers – the picture is distorted by mankind's awkward gender divide, where biological function collides with social realities. The allocation of moral responsibility for contraception might seem neatly divisible on paper, but at the biological level, the odds are against women. We can spill our seed, 24/7, and each magic bullet can be as effective as the last, but if one hits the spot, women have to deal with it, one way or another.

You can blame a reckless and feckless man for running out, but you can't blame anyone for the mechanics of the situation, not that pesky patriarchy, not even the Romans, it's just the way it is. Elliott appears to accept this – hence the suggestion that sisters will keep on doing it for themselves; while responsibility should be equally divided, in practical terms it is women who face the greatest impact from an unplanned pregnancy, and therefore they who will continue to take precautions.

This biological imbalance goes deeper of course – and the social and personal implications deeper still. Responsibilities around contraception inevitably merge into responsibilities regarding conception, and suddenly the apparently clear-cut requirement for men to take their fair share of responsibility seems problematic. Men are expected to take responsibility for a child if things pan out that way, yet at the same time, they have no rights regarding the conceived child.

If women want men to, as it were, contract to care for any conceived child, then natural justice would appear to demand some reciprocal rights. Yet women seem to be extraordinarily resistant to delivering on this side of the bargain. It's hard to imagine many feminists demanding that men get a say in a decision on abortion, for instance. Hardly surprising – as granting a man a veto on aborting his child simultaneously condemns a woman to carry an unwanted baby – a monstrous imposition, I agree. Yet the opposite position seems to me equally wrong.

This looks like a paradox – and I don't like paradoxes. I generally find if you come across one, then your underlying assumptions are wrong. Here our assumptions are essentially about the equivalence of the sexes, and about that equality of responsibility. To break that paradox – to morally recognise only the woman's rights in that situation – it seems to me we need to reject male rights and, with that, we must surely reject male responsibility.

It's an awkward conclusion and one I'm not at all happy to draw. Sex can be spontaneous and chaotic – gladly – but sex that can lead to pregnancy never should be. Unplanned sex fine, unplanned pregnancy, not. And I have nothing but contempt for a man who abandons his child. But should I still take this view? The moral framework we are living in today demands responsibility from men but offers nothing back – and that's an injustice, however you cut it. It seems to me that we need to either accept that men shouldn't need to take responsibility for unwanted children, that they shouldn't be expected to take responsibility for contraception, or we need to figure out some way that they can share in rights over the child, planned or unplanned, born or unborn.