It was such a small reference and it wasn't really relevant to the plot so it didn't mean anything to me. I think he just did it because "haha imagine if Nixon met a gay hero who wanted to marry a black man, how crazy would that be!?" Nothing special there.

Iceglow, regarding the snog: (SPOILER)Remember how River Song mentioned at some point that every time she meets Doctor Who, they never share memories, and how it's 'in reverse'? This means that she's effectively reversing her history with him. When the Doctor first meets River Song, she dies. In that episode, she can't tell him how she knows him because he can't be told about his future. "Spoilers" she says. The Doctor meets her again later, and recalls her from their previous meeting. Nothing more. In this episode, they snog - she starts it, he's confused and carries on. She is mortified that this is his first kiss with her. Why? Because that, for her, means it's her last kiss with him. In later episodes, the Doctor will slowly go through his life in reverse to hers. Every first for him is a last for her, every last for him is a first for her. So eventually, she will meet him, they will kiss, and that will be her first kiss with him. He will know this means it is their last kiss in his timeline. Later, he finally meets her and she doesn't know who he is, he will know that he will never see her again. Personally, I suspect that they will have a full relationship, probably including marriage. How else would she know his real name, when even Rose did not?