From a 2004 study:
"Abstract
A postal survey was conducted of members of the Association of Forensic Physicians (UK) to determine whether accidental anal intercourse occurs in heterosexual relationships and, if so, whether intoxication by alcohol or drugs and sexual inexperience were likely to be causative factors. Of the 512 (47.9%) replies, there were 498 individuals who had had a previous heterosexual relationship and may have experienced accidental anal intercourse. Of these, there were 26 (7.2%) males and 14 (10.4%) females who reported at least one lifetime episode of accidental anal intercourse. Amongst those with a history of accidental anal intercourse, 79% reported that they were sexually experienced at the time and 83% reported that their partners were sexually experienced. Personal intoxication by alcohol or drugs at the time of accidental anal intercourse was reported by 43%, with 41% reporting that their partners were intoxicated.
Conclusion
This survey found a lifetime incidence of at least one episode of AAI in 7.2% of males and 10.4% of females. Comparison data between the sexual practices of the study group and the general population, together with theoretical considerations of possible selection bias, suggest that these figures are likely either to be representative or, perhaps, an underestimate of the incidence of AAI amongst adults in the UK.
Although this study did not set out to examine how accidental anal intercourse occurs, it does provide anecdotal evidence that sexual position and the use of lubricants both predispose to it. Further research would be needed to establish how important these factors are in AAI.
Intoxication appears to be a factor in less than 50% of cases of AAI and sexual naivety is relatively uncommon, the evidence suggesting that AAI is more common amongst those with more 'adventurous' sexual lifestyles"