Understanding and respecting boundaries is one of the key ingredients to having a healthy sex life and relationship. However for many people, especially women, saying "no" can be extremely challenging.
There's not a human being alive who hasn't had their boundaries crossed at some point in their life. Just spend a few minutes watching the interaction of a parent and a toddler having a temper tantrum and you'll see what I mean.
Those of us who have had our boundaries consistently crossed in childhood (whether from overbearing parents, being constantly criticized or experiencing some form of abuse), have a harder time maintaining boundaries as an adult.
We are "people pleasers" believing that if we just say "yes" we will get the love, appreciation and connection that we crave. If we say "no" we may be faced with rejection and abandonment, further lowering our self-esteem.