Sexual Ethics in Buddhism I have a couple of questions regarding the Viewpoints of sexuality in Buddhism for lay people.


Why is lust seen as being unskillful? Lay-people are lay-people because they don't want to renounce worldy life; they strive for 'heavenly realms' instead of nibbana. What's unskillful in a natural urge? Non-engagement won't decrease the urge either.
In a sutta the Buddha said the consequences of an action are important, and this is the reason why he abandoned thoughts of sensuality, because it harms others and himself. It obstructs wisdom (this is true because it hinders Nibbana), but why are sensual/sexual thoughts dangerous? If I lust after a woman in a healthy, non pathological way, I see no danger there. As long as I don't sexually harass her? Maybe she has sexual fantasies as well?
Samadhi is 'achieved' by the momentarily stilling of the 5 hindrances. Elsewhere it was stated that past sankharas influence present sankharas, and present sankharas in turn condition future sankharas. If past sankharas were 'kama chanda' it is likely that I engage in the present in such a manner. Suppose I do, isn't it possible to cleanse present sankharas during meditation (after some while) in order to concentrate the mind, resulting in seeing things as they are ('vipassana)?
Why is lust seen as being unskillful?

Your question goes directly to MN1, which is a difficult sutta:


  relishing is the root of suffering.


Briefly, the delusion of lust assumes that one can separate lust from the suffering and loss that follow. In other words, one might ask "how can this good feeling cause suffering?"  But look at how much effort we devote to peculiar practices such as "maintenance sex" or "faking it". Seeking the eternal, we might try to immerse ourselves in lust. But it always ends. We can never escape the suffering. It is unskillful because the more you lust the more you suffer. Do addicts look happy or lost in suffering?

The urge is real. It is a biological chemical imperative to procreate honed to compel over generations of successful breeding. And when followed heedlessly leads to overpopulation. Which means that we should, perhaps, skillfully consider that very urge as "not my self". 

If I lust after a woman in a healthy, non pathological way, I see no danger there.

The danger is subtle. Women will have friends, some "pretty" and some "un-pretty". And will you be kinder and more devoted to the pretty woman and less kind and devoted to the un-pretty friend? That is the trap of pretty. That we should blindly value someone for the shape of their skin rather than the clarity of their heart.


  Seeing a sight with the eye, you linger in the neighborhood of a sight that’s a basis for happiness or sadness or equanimity...Therein, by relying on the six kinds of renunciate equanimity, give up the six kinds of renunciate happiness. --MN137


This will take time...

isn't it possible to cleanse present sankharas

Your question is on detailed practice with a cleansing focus. However, it might help to inform that practice with a broader knowledge of vital conditions. Also, since your inquiry is based the mechanism of delight, you might find it interesting to explore the exact point where the hook of delight sets itself in the middle. Practice might proceed better with more focus on relinquishing and not getting hooked.