I wonder why anal sex is different? I think most laws of around consent are
supposed to be based on "maybe maturity level, etc" so, if when ever they wrote the law, they decided that (in this case female) was "mature" enough to consent to vaginal sex but not mature enough to consent to anal sex...
This clearly wouldn't take into account a young gay male... not able to consent to have sex until 21...
And, for no particular point, there are many cases of young (religion based) females that wanted to preserve their vaginal virginity for marriage, engaged in anal sex instead.. or so I've heard
Sorry for not really adding any info to this.