是现在分词短语作定语修饰宾语 people,
这里的and有两层意思,后半句既是独立的一个意思也是前半句的延伸,相当于
it's just two people going out to dinner;
it's not two people going out and then having sex.
定语从句It's just two people who go out to dinner;
It's not two people who go out to dinner and then to have sex.