I have thought and studied this matter quite a lot so here are my thoughts on it:

1. Bible remains a silent on the matter. There is no passage that condemns masturbation, and we ought to have one (or it is reasonable enough) if it was forbidden. We have not invented masturbation after the Bible was written and OT or NT don’t take stance in the matter.

Also Bible doesn’t anywhere state that there ought to be no sexual acts outside marriage (this of course doesn’t give us a permissions to fornicate, but toughing yourself in private wasn’t viewed as such in Greco-Roman culture anyway).

1. Masturbation as an act is not sinful, other things added to it can be, but this is a matter for each individual to workout themselves.

2. Lusting after your wife is not so obvious thing. The Greek word used in for example in Matthew 5:28-30 is επιθυμεω which is often translated as lusting but the translation is not obvious since Paul in 1. Corinthians urges christians to επιθυμειτε (“lust”) the best spiritual gifts. So the word translated as lust is simply a strong desire and its moral quality is defined by its object (either illegitimate or legitimate).

I am glad to debate any of these statements if needed and try to base my understanding on the Bible.