Kaam is of 4 Purusarth namely Dharma ,Artha , Kaam and Moksha.
Gita 16:21
There are three gateways to the hell of self-destruction for the soul. These are Kaam, anger, and greed. Therefore one should abandon these three completely.
Krishna says to abandon Kaam completely. If we abandon it completely , then how it is purusarth. And if it is Purusarth , then how it leads to hell ?
You have misread the verses probably.
Let's start with the Sanskrit Verses.
चिन्तामपरिमेयां च प्रलयान्तामुपाश्रिताः ।
कामोपभोगपरमा ।एतावदिति निश्िचताः ॥ 16.11 ॥
11 Giving themselves over to immeasurable cares ending only with death, regarding gratification of lust as their highest aim, and feeling sure that that is all.
English Translation by Swami Sivananda
The keyword is 'paramā (परमा)'.
So, Sri Krishna is admonishing those people who tend to think that "the enjoyment of desirable objects, sense gratification, lust etc, as the highest human goal".
Let's see this in the light of commentary by some preceptors -
English Commentary By Swami Sivananda
They are beset with immense cares? worries and anxieties and their minds are engrossed in aciring and preserving the countless sensual objects. They have got the strong conviction that the sensual enjoyment is the highest end of a man. They are steeped in enjoying the objects of the senses. They firmly believe that that is everything. They believe that sensual enjoyment is the supreme source of happiness and there is no such thing as eternal bliss of the soul or transcendental bliss of the Self. They have no belief in the happiness in another world (or plane) or in the perennial bliss which is independent of sensual objects? which is beyond the reach of the senses. They have a dull and gross intellect? and so they cannot grasp the subtle higher truth. Sensual enjoyment is the greatest object of attainment for them.
Thus people who give-in to their evil resolutions and desires, which only end in death; and those who seek only the gratification of desire as the highest goal; seeing nothing beyond that - it's those ilk being chastised here, not the puruṣārtha of Kāma.
The god is not demonizing the four Puruṣārthas that are based on Dharma. As I already discussed in this answer
यस्मिन् धर्मसमायुक्तावर्थकामौ व्यवस्थितौ इह लोके सुखी भूत्वा प्रेत्यानन्त्याय कल्पते ॥ २.५५ ॥
A person, in whom Artha and Kāma live side by side supported by Dharma, becomes happy in this world and becomes entitled to infinite nature (eternal salvation) after death.
Kurma Purāṇa I.2.55
To conclude -
So the Puruṣārthas of Kāma and Artha aren't to be abandoned, they're to be practised in the dharmika ways only, with faith and belief in the almighty.
Lust has to be abandoned completely for people who are struggling for moksha. Kama as purushartha is not a license for sexual pleasure but to keep the human race alive.
Dharma
Dharma, understood as the Supreme Good of man, can never have wealth as its reward. Nor has wealth, understood as an aid to the achievement of Dharma, been conceived as an aid for the attainment of kama (sexual desire). Kama in its turn is not a call to indulgence in sexual pleasures, but a mere inducement implanted by the Creator to make life unbroken. And as for life, it has the quest of the Supreme Truth as its end – not certainly Karma (i.e. performance of rituals and social duties) for the attainment of wealth (prosperity here and heaven hereafter).
Srimad Bhagavata Purana I.2.9-10
