A lot of us are less than well-informed about how to have safe sex and prevent pregnancy as trans* guys, partially because there isn’t a lot of information out there specifically for us.
-For oral sex involving vulvas (vulvae?), a good way to reduce the chance of STI transmission is to use dental dams. These can be purchased at sex shops and similar locations or created using saran wrap, condoms, latex gloves, or other materials. They are also sometimes available for free or very cheap at university health centers, Planned Parenthoods, and LGBTQ resource centers.
-If you’re using a prosthetic/dildo for vaginal, oral, or anal sex, it is important to make sure the materials it is made of are safe and that it is kept clean. Certain materials are more likely to grow bacteria than others, and certain materials contain phthalates, which are chemicals used to soften PVC and may be dangerous (a good rule of thumb to tell if a prosthetic contains phthalates is if it has a greasy texture and/or smells strongly of plastic). Prosthetics that are labeled non-porous and phthalate-free are generally safer, but that doesn’t let you off the hook for regular cleaning. Different materials require different cleaning methods, and so it’s good to do your research. If you are unsure if your prosthetic is clean and free of harmful chemicals, using a condom can help make things a bit safer for your partner. As a side note, if you suspect that your packer may contain harmful chemicals, you may want to wear it so it is not directly against your skin (for instance over your underwear or in its own little sock).
-Testosterone may stop you from ovulating, but it probably isn’t a good idea to use it as your only form of birth control. Hormonal birth control while on testosterone is not really an option, so the pill, patch, and shot are out. However, condoms, diaphragms, and non-hormonal IUDs are still effective options for guys on T (this is not a complete list of all methods, and not all methods are created equal, look up effectiveness and weigh the pros and cons of each method). Hormonal birth control is fine for guys who are not on testosterone, although many trans* guys are uncomfortable with some of the “feminizing” effects hormonal birth control may have (for example, breast enlargement or breast tenderness).
For more information on safer sex and contraception, check out Planned Parenthood’s website (unfortunately pretty much geared toward cis people but still pretty helpful), MegaThatcher’s video on Safe Sex for Trans Guys, and Queertransmen.org.