Oral and anal sex and their effects on conception are a highly debated issue in fertility forums. Some studies indicate that you can build up immunity to your partner's sperm, making your body reject it upon contact. The theory is much along the same lines as allergy shots. Continued exposure to a substance can build up antibodies that later fight it. So, if a woman swallows the same man's sperm, she will build up immunity to it. Some people oppose this view for oral sex but support it for anal sex. They claim if sperm is ingested, it gets broken down and doesn't produce antibodies. They do agree, however, that anal sex can result in antibody production against sperm. The key is the sperm entering the bloodstream, which is likely in anal sex. It's also likely with oral sex if there's an open wound in the woman's mouth. Even though the evidence is inconclusive, it might be better to be safe than sorry if you're trying to become pregnant.