First of all there is No sexuality building up..It's only a chemical reaction in the brain much like happiness and sadness,it's all in your head,and it only gives you the impression/illusion that is building up because you are focusing on it.
The more you focus on something the more it builds up..like anger you magnify it.
Always remember that the sexual feeling will pass but due to it's strong nature it might seem like it won't until you give in.For this reason you need to implement a few methods to make it subside naturally.
1. Meditate on the foulness of the body. Think about everything revolting like blood,feces,pus,bacteria etc..
2. If there is a particular person you desire think about all the revolting things in her/his body like i mentioned above.
3.Meditate on your own body.So that you'll realize you are just skin,nails,hair,etc.
4.Look at images of people who were once beautiful but who have now grown wrinkled,crippled and old.
5.Read Dhamma books to take your mind off sex.
6.Write a letter to yourself when your not feeling sexual and read it when you face this issue.Remind yourself of Nibbanna and how important it is to realize it.Think about the sacrifice The Buddha had to make to reach it. Nothing great can ever come without sacrifice.Also write down all the consequences of craving for sensual pleasures and all the benefits of surpassing it.
7.Realize how important it is to train in morality,to keep yourself pure.And to always guard this purity 24/7.Most people don't have a problem with sitting down to meditate or read a lot of dhamma books but can you imagine how rare it is to have someone master their own actions.Be that person!
8.Remember that if you can't train something gross like the body then how do you expect to train something subtle like the mind.
9.Always be mindful.
10.Anapanasati..breath meditation...this is by far the most effective technique because when you are doing this meditaion you are not trying to stop thinking about sex,you are not trying to restrain yourself,that's the thing..your not trying at all..your mind is completely focused on your breathing your not thinking about anything else..not about dinner,or tv,or doctor's appointmet or sex..nothing..just breath..that's the only thing thats exists.By the time your done,trust me, "you won't be in the mood."
11.Last but not least..just say No and don't do it.There's a million reason why you want to.
But don't.Simple.Nobody else gonna do it for you.Your life your call.
I hope this helps