That's one of the first things that we (I'm a Survata co-founder) noted in seeing the data too.
I see a few possible explanations:
(1) A woman who sexts could have multiple sexting partners. In the extreme, you could have every man in the world sext with one woman, making the male sexting prevalence 100% and the female near 0.
(2) While we defined sexting as "sending or receiving", some respondents may interpret the question as primarily about sending. There could be a gender bias in the sending vs receiving of sexts.
(3) As you point out, the data is reported behavior and not observed behavior. Reported behavior often is a good proxy for observed behavior, but it is not perfect. And there are known to be effects where certain demographics answer questions dishonestly for conscious or unconscious reasons. Perhaps women are less willing to admit to sexting behavior.
I see a few possible explanations: (1) A woman who sexts could have multiple sexting partners. In the extreme, you could have every man in the world sext with one woman, making the male sexting prevalence 100% and the female near 0. (2) While we defined sexting as "sending or receiving", some respondents may interpret the question as primarily about sending. There could be a gender bias in the sending vs receiving of sexts. (3) As you point out, the data is reported behavior and not observed behavior. Reported behavior often is a good proxy for observed behavior, but it is not perfect. And there are known to be effects where certain demographics answer questions dishonestly for conscious or unconscious reasons. Perhaps women are less willing to admit to sexting behavior.