| We're so glad you came|
|Reach around the subject|
Practicing safe sex
In addition to "safe" essentially being a euphemism for birth control, it also implies a way of avoiding STDs - hence the use of condoms as well as pill-based birth control methods. It can also imply, although this isn't widely used, that the sex is physically safe (no copulating in the middle of a base jump, for instance) or done with full informed consent. This is the core of most sex education that isn't moralising and misleading.
Practicing "safe" sex
Republicans, and most on the conservative and/or religious right, tend to define safe sex as purely abstinence. Indeed, this is the safest form of sex, although this isn't saying much as it is like redefining a famine as "safe cuisine" or atheism as "safe religion" (zing!). The other down side to this method is that the education offered in this form of "safe" "sex" is usually abstinence only. In this form of sex education the information on actually carrying out safe sex is either taught badly, in a deliberately misleading way, or isn't taught at all. This means that should sex actually happen — for whatever reason, kids will mess around whether you like it or not — the methods used will be very unsafe. This includes substituting vaginal for anal sex, completely neglecting condom use, and often perpetuates bizarre myths about how girls can and can't get pregnant.
Antidepressant effect of sex fluids
Women who have regular unprotected sex are less depressed and perform better on cognitive tests compared to women who abstained from sex or those who had the same amount of sex but used condoms. Semen contains several antidepressants: cortisol (known to increase affection), estrone (which elevates mood), prolactin (a natural antidepressant), oxytocin (also elevates mood), thyrotropin-releasing hormone (another antidepressant), melatonin (a sleep-inducing agent) and even serotonin (perhaps the most well-known antidepressant neurotransmitter). 
Obligatory "safe sex" joke
Ms. Kensington "Well, did you use protection?"
Austin Powers "Yes of course I did. I had my 9mm semi-automatic."
- Excerpt from Austin Powers: International Man of Mystery