One of the play's main themes is the relationship between love and marriage. At the time when Aphra Behn wrote the play—in Restoration England—marriage was largely seen as a social or religious custom, or perhaps a kind of glorified business transaction between aristocratic families. Love seldom, if ever, entered into the equation.
Hellena and Willmore are unusual in this regard in that they want to get married for love. They are both very much free-spirits and always follow their hearts rather than the dictates of established convention. But of course, there are limits to their freedom in such a restrictive society. Though strictly speaking they don't need to express their love through the formal bonds of holy matrimony, they choose to do so anyway. Hellena and Willmore may be free spirits, but they still have to live in society, which means that they must compromise in some way with prevailing standards. In any case, marrying out of love is a sufficiently subversive act in that it shows how it is possible to maintain social respectability while still remaining true to your heart.