What is their source?
1 Peter 1:4 Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
Fleshly lusts are roughly divided into 3 kinds.
1 John 2:16 For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world.
These are termed the ‘works of the flesh:
Galatians 5:19-21 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, 20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, 21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
These come from within
Mark 7:21-23 For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, 22 Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: 23 All these evil things come from within, and defile the man.
It is our fleshly desires that cause us to sin
James 1:14-15 But every man is tempted, when he is drawn away of his own lust, and enticed. 15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
In Christ, we are to deny ourselves (not give in to our lusts) and live by Christ’s commandments.
Mark 8:34 And when he had called the people unto him with his disciples also, he said unto them, Whosoever will come after me, let him deny himself, and take up his cross, and follow me.
Titus 2:11 For the grace of God that bringeth salvation hath appeared to all men, 12 Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world;
I hope you find this helpful.