I think the impact of JW supposed rules on sex in most marriages over the past 20+ years has been minimal. I know no one who really thinks oral sex is banned. I know no one who really thinks that there is only one position or is not adventurous in some way. We knew an ex bethelite couple who had a wardrobe full of sexy role play dressing up clothes.
My wife and I have been faithfully married for a long time over 2 decades now. We both followed WT doctrine as far as sex before marriage was concerned. I can honestly say that the elders or the Watchtower have never had an say in how we behave. We enjoy a really good time and that's because we love each other and have worked hard to have a successful marriage. At the risk of being shot down I do think some of this is due to principles and practical guidance that we have both tried to apply as Witnesses. I am not saying you a happy marriage is only possible by being a Witness, just that even in my mentally out state I do think there was perhaps something positive from my time in.
No doubt some do have sexual hang ups or do not enjoy a healthy sex life. In my experience this is not thanks to WT doctrine on sex in itself but down to problems people either have with themselves or with each other far beyond what goes on in the bedroom. There is no doubt that there are a significant number of Witnesses who remain in unhappy marriages. I doubt very much however that this is because of some hang up what goes on in the bedroom. I think it is because they got married too young, too quick to the wrong person. Their goals diverged. The outlook diverged. There aspirations and hopes were not met. Keeping their life "simple" brought money worries and dissatisfaction as they could not do the things others were doing. The marriage was too hierarchical with a bright wife forced to be in subjection to a dim husband. They lost respect for themselves and each other.
Problems in the bedroom are generally the symptom of bigger problems elsewhere, not the cause. I am 100% sure that many of the Witnesses in happy and successful marriages are enjoying lots of things in the bedroom that would have given Freddy boy a coronary.