I am going to share my experience about the same, and expect people not to make fun out of it, as it can be the situation with so many of us.
It depends on various factors man.
Sometimes you don't have time to think about sex and the same happens with your partner.
Sometimes you just don't want it. It's all about mood. Again the same applies to your partner.
Even more difficult situations comes is, when your partner is not too keen about having sex, though he or she enjoys it when it happens. It's like, sex is just not their things.
Now a days, these smart phones and similar gadgets, being active on social platforms to make their presence feel also makes the person super busy or away from even thinking about sex.
Or may be just you have watched too much porn, and your imaginations went ahead of what the actual scene is.
Similar is the case with me. I am very keen about sex. I like it a lot. Like seriously, A LOT. But my wife isn't too keen about it. She loves when it happens, but at other times she just becomes very angry if I touch her boobs just like that.
I also had watched too much of porn, and expected a lot from my partner. She wants to experiment, but ends up having just a missionary sex, though that's so good.
Now this has taken a bad turn. I am looking out for sex. I lurk at almost every girl out there who I feel looks sexy, exposes sexy part of their body like sleeveless arms, bust area and all.
It's gone to such an extent, that I feel like having sex with my office colleagues who are very sexy, though they are so good friends of mine. I keep on looking at pics of my college girls, some of my relatives such as my sis in law and masturbate quite often. I have also met couples and girls outside, and backed out every time due to the love for my wife.
Now for you:
Either don't think too much about it, or else talk to your wife for it. Try to spend time with her, and don't think about sex all the time. There are many other ways to reach to sex such as spending quite quality time in each other's arms, talk about both of you, about your bodies. Love her, caress her, else it can be late
Hope you are able to make out today
Masturbating is totally okay in marriages. Enjoy it alone, not too often, when you need it too much and your partner is not in a mood.