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.

In Chapter Two of The Communist Manifesto, Marx and Engels make some important point about marriage and women in the bourgeois world. First of all, they argue that a woman is nothing more than an "instrument of production" in bourgeois society. In other words, through her ability to bear children, she is an extension of the means of production because she can produce the next generation of capitalists and workers. In addition, being an "instrument of production" also means that she is exploited for her labor, both in the home and in the workplace.

Marx and Engels also claim that bourgeois marriage does not benefit women at all. Because all women are viewed as property, bourgeois men are not faithful and routinely "seduce" other men's wives. In other words, the bourgeois believe that they have a right to a woman's body because she is an object and marriage legitimizes this view instead of abolishing it.

For Marx and Engels, then, marriage is an exploitative institution for women.

According to Marx and Engels, marriage in bourgeois, capitalist society is a sham.  It is supposed to be sacred, but it is in fact simply another way for one kind of people to exploit another.

Marx and Engels say that the bourgeois do not really value their wives.  Instead,

The bourgeois sees his wife a mere instrument of production.

They see their wives as mere instruments, not as people to be valued for themselves.

In addition, Marx and Engels charge that bourgeois men treat women as sexual objects who are fair game for anyone.  They charge that the bourgeois men feel that they can have any woman they want.  They say

Our bourgeois, not content with having wives and daughters of their proletarians at their disposal, not to speak of common prostitutes, take the greatest pleasure in seducing each other's wives.

Marx and Engels, then, see bourgeois life as hypocritical.  The bourgeoisie claim to love their families and believe in fidelity but instead they treat their wives as means of production and all women as their sexual objects.