Why do Mitch and Cam never kiss?

The answer is probably "viewer comfort".

I'm a straight guy (bear with me, I'm explaining viewer experience) and while I strongly agree that gay partnership is acceptable, I still cringe when I see a kissing scene between two men.

The fact of the matter is: Society has gotten used to seeing a man and a woman kiss on TV, but non of us grew up seeing men kiss, and while most intelligent people will respect the rights of people of all sexualities, kissing scenes between two men can be cringe-worthy for straight guys.

While the franchise is taking big steps by including homosexuality in a tv show, it still has to please viewers, and many viewers are simply not comfortable with gay kissing scenes yet.

Note: I hope I'm not offending any gay readers here at all.

As @jt0dd says, it seems to be viewer comfort.

A Facebook petition was actually set up by a group of fans back in Series 2 calling for a kiss between them, as it hadn't happened yet:

"Cam & Mitchell, the adorable gay couple on ABC's Modern Family, have not been shown sharing even a brief kiss throughout the series' first hit season. ABC isn't afraid of gay characters, so why won't they let them show some love?" the group writes as its credo.

This was just one example, but there were a few different sources calling for this moment. It did finally arrive, with Vulture stating:

The kiss was both frustratingly and admirably understated. On the one hand, seriously, that's what you call a kiss?! On the other, Modern Family's creators didn't bow to the pressure to make Cam and Mitchell's kiss a huge deal, sacrificing character and story line in the process.

Given the amount of attention and hype this one little kiss generated, it's easy to see why the producers have avoided giving it any more exposure. Whilst they may be happy with gay characters, it's senseless for them to do something that risks alienating any of their fanbase. If their market research has suggested that having more intimate scenes between Cam and Mitchell could lead to a loss of viewers who find it distasteful, or unpleasant, to watch, they won't do it, regardless of their own opinions on the issue.

This can be clearly seen in Modern Family in any intimate scenes that Mitchell and Cameron find themselves in, compared to the other heterosexual people on the show. Whilst the others get far more on screen romantic and sexual scenes, Mitchell and Cam's are greatly reduced.

This does largely appear to be an attempt to appease a fanbase who, whilst supportive of their relationship, don't wish to know or see the more intimate aspects of it.

Obviously, this raises questions about whether this is fair or whether, in time, the show should do more. These questions go beyond the show though, and venture in to the realm of what mainstream America is actually prepared to watch. There will always be campaigners in favour of more graphic scenes between the couple, and others in favour of less. Time will tell how the show goes, but for now it's treading a happy medium where it's trying to portray the love and care between them, rather than anything more graphic or sexual.

This precise issue was addressed by ABC in a statement. The very short answer is that in-universe, Mitchell is written as being uncomfortable with public displays of affection.

Out of universe (and presumably to assuage viewer's concerns) they wanted to stress that upcoming episodes would include an in-depth exploration of the depths of their relationship, include both kissing and cuddling:

"Cameron and Mitchell are a loving, grounded, committed, and demonstrably affectionate couple and have been from the beginning of the series. It happens that we have an episode in the works that addresses Mitchell's slight discomfort with public displays of affection. It will air in the fall and until then, as Phil Dunphy would say, everyone please chillax."

and in this interview with the showrunners

[On the upcoming kiss between Cameron (Eric Stonestreet) and Mitchell (Jesse Tyler Ferguson):]

FERGUSON: That was something that [the writers] had in the works very early on; unfortunately it’s going to look like a response to the Facebook campaign. Some people are complaining that it shouldn’t be a big deal and it’s being made such a big deal. The thing is, it never was even meant to be a big deal. I think when people see the episode, they’re going to understand what our plan was the whole time…. We’re handling the characters very carefully. We’re in a lot of living rooms every week and it’s fantastic. And I don’t want to scare off the people who are just getting comfortable with Cameron and Mitchell…. It’s very simple and it’s not a big deal. That’s exactly what it should be.

STONESTREET: It’s about Jay and his lack of affection and where Mitch learned that. And it’s very real. We think it’s smart and perfect.… Hopefully everybody will see that we just want to tell truthful, honest, sweet, believable stories that aren’t driven by anything political or motivated by a movement.

Viewer comfort is probably the answer, but I'll also point out that the actor who plays Cam (Eric Stonestreet) is straight in real life. He may have some influence on how often he and Mitchell (Jesse Tyler Ferguson, who is gay in real life) kiss or show other physical affection.

I think it may have to do with viewer discomfort but in addition, the actor who plays Cam, Eric Stonestreet isn't gay so he may not feel comfortable making out with his male costar. I don't see why this should be because its just acting and gay actors kiss women all the time on T.V...but in any case, this may be a contributing factor.

Of course I'm sure viewer's comfort is a factor, but you're forgetting what it's like in the show. Mitchell's dad is obviously very uncomfortable about him being gay and kissing another man. Jay's friends are there and so is the entire family. I'm sure a lot of them would have been uncomfortable too

It's because Eric Stonestreet who plays Cameron is actually straight so I think he finds it a little weird.