Islam.SE an "Islamic" site in the sense that we learn about Islam here---it's not an online mosque.
However, I agree that there's many questions about the OP's masturbation, fornication, ultra-specific sex acts, etc., which are both off-topic (counselling or peer support; and often don't have an attitude of detachment) and off-putting to the users. Furthermore, such questions encourage other such questions.
Keep in mind that not all "adult" posts are like this. (Think pornography vs. sex education.)
A partial solution is to ignore tags; see Did you know you can ignore tags?. But it doesn't really stop the inflow.
We can discourage these posts e.g. by:
We can downvote.
We can vote to close (or flag to get others to vote to close). Ideally we should be getting close votes within minutes of them being posted.
We can comment to explain one's personal "adult" issue is not an appropriate topic for the site.
Don't answer them; the users probably don't care about the site and "reputation", but they care about the answer.
Keep the front page looking professional by improving question titles, so these questions look out of place.
Keep active in producing suitable questions (and answers to questions).
With the example questions given, this is mostly what has happened. If they're closed (not as duplicate) and downvoted, they'll end up automatically deleted.
(Recently I've been editing away the haram tag; perhaps bumping these questions has encouraged this behavior, and it may not continue after the retagging has finished.)
I think we should have a question sandbox for all the new questions from new users and it should be gone through it before appearing in the main site.so that we will have only good quality posts.
We have the Review queues for this purpose. (First Posts---Review first posts from new users.) StackOverflow has Triage and Help and Improvement queues to help with this problem, but it's not network-wide.
We're not capable of changing the site mechanics here, that's what meta.SE is for. (But I don't recommend making a feature request there until you're very familiar with the StackExchange software [etc.].)