Would we miss the opposite gender if they didn't exist?

It would be impossible for us to survive if the opposite gender didn't exist as we wouldn't be able to procreate.  So in that sense, we would certainly miss the opposite sex if they didn't exist.  There are a fair few other things to consider however, and I've outlined them below.

The question at hand takes a completely different form if the opposite gender never actually existed in this hypothetical world. 

If by some event males (I speak from a male perspective) were always the only gender in the world, it would be difficult to imagine that we'd miss females on the basis that we'd never have known about them in the first place.

On the other hand, heterosexual males are born with the instinct to breed with a female, and so there may be some sort of idea in the back of our minds that something was indeed missing from the world.

Equality

In the past, the answer to this question may have been entirely different from the one today.  This is due to the evolution of gender roles within society.

Although equality has not been fully achieved, history may have seen an answer along the lines of - "we'd miss women because they can cook, clean and look after the home".  In today's society, men and women are much more likely to share equal responsibility over most roles, and so the emphasis on NEEDING the opposite sex in this capacity is reduced.

Personal preference

Ultimately, personal preference over companionship takes a big role in the answer to this question.  If you're in a happy heterosexual relationship, you're definitely likely to miss your partner if they're suddenly not there.

On the other hand, if you're not interested in any sort of relationship with members of the opposite sex then you're much less likely to miss them.  Of course, I'm sure most people would miss their mothers!