Seems to me that there's no need for the robots to meet to have sex. Obviously in deep space that's going to be a problem, what with distances and speeds. Instead, the robots could quite easily have sex by radio. Sex is, after all, just a way of exchanging genomic data. Let the robot broadcast excerpts from its own design data archive to anybody who cares to listen; let a robot hearing the broadcast patch the input data together with its own design data to produce hybrids.
Certainly this is unnecessary if we're postulating superintelligent machines perfectly capable of redesigning themselves on the fly to meet whatever situations they encounter. But the road to a Culture GSV is a long one. You might well begin with a swarm of rather dumb self-replicating probes with very limited capabilities - I mean, somewhere down the line there must have been an intelligent designer, and so the progenitor robot would have had to be incredibly basic. But if you give them the means to exchange design details with each other over long distances - this worked, this didn't - then eventually you might indeed have a horde of sexy, sexy von Neumann machines, all procreating and evolving their way to becoming a galaxy-spanning intelligent race in their own right.
And anyway, even if the robots do not exchange design data - if they don't have sex at all - well, most living things on Earth don't have sex either. Doesn't disqualify them from the 'life' category.