Setting Boundaries with BAE
Before I answer this question, I think it's important to have an understanding of what having a boundary means. Imagine there is a steep hill with sharp rocks at the bottom. If a person were to be driving on that hill, and they wanted to avoid the rocks, logically, the best place to put on the brakes would be at the top of the hill. This is because gravity is pulling the truck down the hill.
Now imagine this hill represents a relationship. On the top of the hill would be physical signs of affection such as holding hands, hugging, then kissing. Towards the bottom of the hill, then, would be sex. The rocks at the bottom represent potential negative consequences of a sexual relationship, such as unwanted pregnancy, or STDs. When a couple sets a boundary, it is just like putting on the brakes. Setting a boundary in a relationship is crucial if one wants to completely avoid negative consequences, or "rocks," in a relationship.
A person could put their boundary anywhere they think is best, however, it's important to understand that the higher you set your boundary, the better chance of success you have in keeping it. This is because of arousal - the "gravity" that pulls people down the "hill", closer to sex. For this reason, the further you go, the harder it is to stop. A place where you could set your boundary could be at kissing. This meaning that you won't go any farther than that.
Every couple is different, but my advice is to have a conversation about boundaries very early in a relationship. Perhaps you are not sure where you want to set your boundary, and you want to make a decision with your partner about how far you will go in your physical intimacy. Approach the conversation with an understanding that setting boundaries high is important for success in sticking to that boundary. With this in mind, discuss with your partner openly about where to set your boundary. You may not agree at first, but be patient and listen to what your partner has to say, and be honest about your feelings with your partner.
Maybe you already know where you'd like your boundary, and you're not sure how to discuss this with your partner. I would suggest that after you've decided where you want your boundary to be, take some time on your own to think it through and make sure that you are willing to stick to that boundary, no matter what. Have a full understanding of where your boundary is and why you want to set it there. Approach your partner and be honest with them. Share the specific reasons why you desire to set this boundary. (Hopefully it's not just, "Because Michelle said so!") Your partner may not agree with where you want to set your boundary, but if they care about you they will be willing to put themselves aside and support your decision. Be willing to answer any questions they may have.
The keys to success in any serious conversation are honesty and willingness to listen. If you or your partner aren't open and honest about how you'd like your relationship to look, you won't truly be on the same page. On the other hand, if you or your partner isn't willing to listen to the other, then there won't be any understanding about how the other feels.
Remember that boundaries are temporary. The goal is to wait until you and your partner have entered into a life-long monogamous relationship, such as marriage, to enjoy all expressions of physical intimacy, without concern about hitting the rocks.