A Dutch study published last month examined the consistent use of condoms within both casual and steady heterosexual partnerships.
2144 individuals were included, reporting 6401 partnerships; 54.7% were female, the median age was 25 (IQR 22–30) years and 79.9% were Dutch. Inconsistent or no condom use occurred in 13.9% of 2387 steady partnerships and in 33.5% of 4014 casual partnerships. There was statistical evidence of associations between inconsistent condom use in steady partnerships and ethnic concordance, longer duration, higher number of sex acts, practising anal sex, and sex-related drug use. In casual partnerships, associations were found with having an older partner, ethnic concordance, longer duration, higher number of sex acts, anal sex, sex-related drug use, ongoing partnerships and concurrency.
Some of the findings aren't particularly surprising—condom use is less consistent when there are a lot of sex acts performed, perhaps due to running out of condoms or a 'well it doesn't matter once in a while since most of the time we use them' mentality. Condom use is also less frequent during anal sex; an act that, while still exposing parties to STIs, can't result in pregnancy and is therefore missing a major motivating factor, if not the primary motivating factor, for use among heterosexual couples.
The link between sex-related drug use and less consistent condom usage is also pretty logical, and so it's the remaining relationship that I find most interesting.
Among both steady and casual partnerships, less consistency in condom usage was reported when partners shared an ethnic background.
I find myself at a loss as I try to consider possible causes for this relationship. Not, of course, that the study proved a causal relationship, so I'm speculating in a very non-scientific manner.
I suspect it could be deeply internalized (subconscious, even) racism. An assumption that you don't want to bring what the 'other' group has back into 'your' people? [Note: uh, that sentence felt pretty gross to type] And maybe there is a lower level of trust with someone outside of your ethnic group initially, but the statistic held true in the steady partnerships as well, where one would think a minimum trust level has been established.
The study may not even be relevant in the U.S., but it presents an interesting point of thought about our own sexual health practices.