Good question. It's not written about much. The agencies that have given me data usually stipulate they don't want their individual numbers to be made public (although its fine if they are aggregated). My survey of agencies includes 25,319 workers. Those in couples (male+female) outnumber singles (individual male or female) by 8:2. Single women outnumber single men by 7:3, generally. Individual organizations may have single women:men ratios of as low as 66:33 and as high as 94:6. Generally, the larger the org, the smaller the ratio between single men and women.
This imbalance can be a significant force for attrition, particularly as people enter marry-and-have-children years. Men will occasionally marry someone on the field, but women most often marry someone from back home, and often leave the field to do so.
I have theorized these numbers in missions actually reflect the situation in a lot of churches, and it's the church numbers that drive the mission agency recruitment numbers. I haven't seen much written on this subject, but if someone has links to articles, or blog posts, I'd be interested in comments below.