Yup, and everybody is happy with this. Till they run up a huge bill and then suddenly they want detailed reports on which sites they visited.

And then Vodacom is lower than snail-***** for not providing it.
But to answer the question; Vodacom does not monitor which sites you visit or shape or restrict your IP session in any way.
With one exception. There are in-line scanners that will look for malicious activities / attacks aimed against you. If these are detected or protocols don't react the way they are expected to do, the firewalls will terminate your session with that site.
And this could well be what you're seeing. If you access the site and it tries to push something down to you that the firewalls don't like, they disconnect you from that site.
But again, it's not because it's an adult site, rather just one that's up to no good.
Also, if ACM (Adult Content Management) is active on your account, you will be blocked from erotic sites. The fact that you say it happens to a number of sites, makes me believe someone (your parents?) have got it turned on.
If you PM me the cell number, I'll check.