Gentlemen, get a vasectomy. But never ever admit to any prospective sexual partner that you have been sterilized.
Sure, you should always be honest about your attitude toward children. And by all means, tell every woman that you don't want kids, that you don't like kids, that kids disgust you. Go ahead and proclaim to every girlfriend that you think of a baby as sort of sexually transmitted disease that cries and leaks shit. Be certain to inform any soulmate that you don't want your freedom compromised by having to raise some diapered parasite. Make it known that you don't want your IQ to decline by being confined in the same room with a babbling infant.
But telling everyone that that you hate children simply isn't enough. Sometimes you need to outsmart the breeders.
Never reveal that you had a vasectomy to any sweetheart. Keep the knowledge about being sterilized to yourself. You are entitled to some confidentiality about your medical condition - and your sperm count is certainly none of your wife's business.
Birth control should always remain a woman's responsibility. The female needs to keep ingesting oral contraceptives not for the obvious biological fact that she is the only one who is that is going come down with pregnancy - but because the routine practice of taking birth control helps focus the relationship on erotic pleasure, not procreation. No matter how successful the vasectomy, no matter that you are shooting blanks, she needs to continue with the hormones. Birth control has an vital psychological dimension: it ensures that sexual enjoyment remains what keeps a couple together, not parenting.
However, no matter how much the woman might insist that she simply loathes children, no matter how vehemently she might shun the little bastards, her female friends will subject her to constant pressure to breed. Her parents will assert that, for whatever convoluted reason, they absolutely require grandchildren. Her peer group will diligently persuade your girlfriend that it is her social duty to become a mother.
And all women will eventually break under such indoctrination. The minute you hear your girlfriend casually assert that the spouse of some acquaintance is such-a-great-father after an "accidental" pregnancy, you have to recognize that your own love affair has come to an end. (It is important to understand that she never actually met the unwilling dad she now claims to admire; all she knows about him is that he hasn't yet strangled the squealing spawn and fled to Venezuela - so in her social circle, that makes him a good parent.) The awareness that your beloved is now saying something positive about childrearing has got to register in your reptilian brain that the months of orgasmic experimentation have ended forever.
Once you hear the "such-a-good-father" routine, you need to be making plans to get out. What you don't yet realize is that she has already stopped using birth control - she just hasn't bothered to let you know. She actually believes that she is doing you a favor by deceiving you into fatherhood.
But when biology doesn't cooperate with her resurgent hormones, she will seek other testicles for that sperm donor. If you don't knock her up, she will inevitably search for another penis that will. There will quickly come a day when she has some "good news" for you; there will come a morning when she does the performance of her "surprised and shocked" discovery that she is pregnant. Enjoy her teary-eyed acting for the last time. Savor that moment, appreciate that instant right before you finally mention your vasectomy. It is not just that the preemptive sterilization will save you hundreds of thousands of dollars in child support, but a vasectomy will give you the levelheadedness to see the passion for what it is. Only a man with a financial death-wish would have sex with a girl who is not on the pill, but only a gullible fool would trust a lady to actually follow the dosage schedule.
Every woman will ultimately betray you; there comes a moment in every woman's emotional life when she considers betrayal her moral duty - not to you, but to "save the relationship." The estrogen-enhanced mind believes that only through betrayal will the intimacy with you progress "to the next level." What you need to keep in the mind is that the secret to a loving romance is to screw her over first.