Typically people have two different sets of boundaries. The first set, often called hard limits, are very clear sexual boundaries that the individual refuses to cross. Another set of boundaries, often called soft limits, can be somewhat more fluid. In fact, many people find their sex lives are even more enjoyable when they push these "soft limit" boundaries and discover what happens when they explore beyond their comfort zones.
Sometimes boundaries are clearly defined and negotiated before a couple engages in sexual activity, such as in a BDSM relationship, but often boundaries are discovered more gradually as a couple gets to know one another.
More About Boundaries
Some people regard sex as a game of boundaries. It’s all about understanding your own boundaries, getting to know those of your partner, and learning when you can push against these boundaries and when you should play within them. In the BDSM community, all of these activities are very ritualized, with clear discussions and sometimes contracts that define each participant's boundaries. Safewords are also sometimes established to help individuals express that a boundary has been crossed. This practice is less common in the vanilla community, therefore increasing the chances of miscommunication, but nevertheless, boundaries are important in all relationships.