Complex sentence: Schwartz and Philip Blumstein concluded that lesbian couples in committed relationships have less sex than any other type of couple (heterosexual married, heterosexual co-habitating or gay male) and that they generally experience less sexual intimacy the longer the relationship lasts.
Split into simpler sentences: Schwartz and Philip Blumstein concluded that lesbian couples in committed relationships have less sex than any other type of couple (heterosexual married, heterosexual co-habitating or gay male). That they generally experience less sexual intimacy the longer the relationship lasts.