The possibility of getting pregnant is always lurking at your vaginal entrance even if you have your clothes on and there is no penetration. If you are not using birth control, all you need is one determined sperm to slip by and reach your egg. Be cautious and prepared.
Chance of Conceiving
Sperm needs fluid to swim and cannot survive when exposed to air. Therefore, the chance of conceiving if you are wearing your underwear is unlikely, though not impossible, depending on the circumstances.
Sperm can't swim through if you are wearing thick panties, nor can you get pregnant if your partner ejaculates in his thick pants. However, there is always that slight possibility that if you are wear panties with a thin crotch and he ejaculates near your vaginal opening, enough semen and live sperm might get through.
In addition, all you need is a shift of your undies out of place, especially if you are wearing a thong, for deposited sperm on your external area to get into your vagina.
In the heat of "making out," risky moments can lead to partial of full ejaculation and accidental access of sperm to your vaginal opening. These include petting, foreplay, and so-called dry humping. It is then possible for sperm to swim in and meet your egg in your tube if your underwear doesn't fully cover your vaginal entrance.
On the other hand, in a moment of distracted passion, your best intentions can disappear, clothes come off, and penetration happens. Even if your partner pulls out before ejaculation, it might be too late because pre-ejaculation (pre-cum) fluid contains sperm.
You might start out with your clothes on, determined not to have vaginal penetration, but judgement can get clouded when things get heated. So don't take chances. Avoid these intimate contacts if you are not prepared to take the risk.
Be prepared for the chance of pregnancy if you decide to engage in any type of sexual activity - clothes on or off. Make sure you have an effective method of birth control. It is also a good idea to insist, even if you are on birth control, that a casual partner is prepared to use a condom to reduce your risk for a sexually transmitted infection.