What does the Scriptures say about guys and girls wearing pants below their backside?
You won't find a direct passage because the style of dress in biblical days is quite different from modern times. However, you can find numerous guidelines which help us understand what is and is not proper clothing for a person to wear.
The primary purpose of clothing is to cover the regions of the body associated with sex. After Adam and Eve sinned in the Garden of Eden, they made belts of fig leaves to cover their nakedness (Genesis 3:7). Yet they understood that these "garments" were not adequate. They hid from God and Adam later explained, "I heard Your voice in the garden, and I was afraid because I was naked; and I hid myself" (Genesis 3:10). From this we learn that clothing that doesn't completely cover the genitals in all positions is not adequate clothing.
Even clothing that would be modest on ground level can become immodest when the wearer is in a different position. The priests were forbidden to use an elevated altar. "Nor shall you go up by steps to My altar, that your nakedness may not be exposed on it" (Exodus 20:26). As a precaution, God had the priest wear linen shorts under their clothing. "And you shall make for them linen trousers to cover their nakedness; they shall reach from the waist to the thighs" (Exodus 28:42). Thus we conclude that exposure of the sexual areas of the body, even though the person generally clothed is still considered nakedness.
Therefore, one answer to your question is that if a person is wearing pants below the buttocks (the backside), commonly called sagging, then that would leave the groin exposed unless they are wearing additional clothing underneath. Most whom I've seen around the neighborhoods wear shorts because they know that if they aren't careful, their pants will fall down at times. However, some tempt fate by wearing only underwear, such as boxer shorts, under their pants. This presents a problem because the fly can open. Even with the brief and boxer-brief styles, the genitals would remain covered, but the tightness of the underwear would leave little to the imagination. The person would not be adequately clothed in these situations.
A second consideration is why people are wearing pants that require the wearer to constantly hold them up. It certain can't be claim to be practical. The main reason I come up with is that this style is being worn for the shock value. Seeing someone wearing pants low makes you wonder how they are staying up, but it also focuses the viewers' attention on the sexual regions of the body. The brief flashes of undergarments, or worse skin, also focuses the viewer's attention on sexual things. Thus, I must conclude that people wearing clothing of this style are sensually minded. "The revelation afforded of an adolescent and/or teenage boy's upper pubic area from the front, and hint of buttocks from behind, by sagging pants is self-explanatory - it is erotic ..." [Chris Chapman, writer on contemporary art and culture].
In I Timothy 2:9-10, Paul gives Christian woman guidelines for their clothing. Similar guidelines would apply to men as well. One of the characteristics given was the Greek word aidos, which means having a sense of shame or modesty. In other words, the person should select clothing that wouldn't cause them or others to blush to see them. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5). Christians are not to be advertising their bodies.
Questions and Answers regarding Clothing
Articles about Modest Dress
Sermons on Clothing
"Proper Attire" from Growing Up in the Lord: A Study for Teenage Boys
"Proper Attire" from Growing Up in the Lord: A Study for Teenage Girls