In the Communist Manifesto, Marx and Engels say the following about marriage:
Bourgeois marriage is in reality a system of wives in common and thus, at the most, what the Communists might possibly be reproached with, is that they desire to introduce, in substitution for a hypocritically concealed, an openly legalised system of free love.
If we unpack this, what Marx and Engels argue is that most bourgeois men sleep with women other than their wives, and thus have established "a system" of common wives. Marx and Engels contend that although the communists are attacked for advocating free love as a replacement for marriage, this attack is hypocritical, for free love (at least for men) is already the existing practice in the bourgeois state.
They also argue that it is self evident that the prostitution which is so common in the bourgeois world would be abolished under communism.
Marx and Engels state the following:
The bourgeois sees his wife as a mere instrument of production.
While bourgeois men, Marx and Engels say, understand the "community of women" advocated by Marxism as another form of exploitation, these men don't understand that the Marxists actually wish to abolish the idea of women as mere instruments of production and restore them to a fuller humanity.