I think the adultry scripture you speak is Matthew 5:28:
But I tell you that anyone who looks at a woman lustfully has already committed adultry with her in his heart.
Adultry being a sin, of course, Jesus is basically saying that if you’re looking at women and thinking if sex you’re in sin. Seeing as how you can’t very well commit adultry with your own wife, looking at any woman except your lawfully wedded wife would be a sin. I assume one looks at a woman lustfully before getting into premartial sex with her.
For another example, look at Galatians 5:19:
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery…
And Ephesians 5:3a:
But among you there must not be even a hint of sexual immorality, or of any kind of impurity…
And Colossians 3:5
Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed…
From what I’ve heard (and I’m no scholar, but it makes sense), sexual immorality is basically sex outside of marriage. Sexual impurity is lust (see above). I do doubt that either of these terms is expressly adultry in the “cheating on your partner” sense since Paul could have just as easily said ‘adultry’ instead of using more all-inclusive terms.
Finally, I Timothy 5:1b-2 5:11:
Treat … older women as mothers and younger women as sister, with absolute purity.
As for younger widows, do not put them on such a list. For when their sensual desires overcome their dedication to Christ, they want to marry.
The former is pretty obvious in how men are to treat women. The second is a warning not to put young women on the list of widows (who don’t remarry but rather dedicate themselves to Christ), for eventually they’ll desire physical comforts and would be sinning if they start sleeping around (it also goes into them gossiping, etc).
Ok. That’s enough of that. Allow me to pre-empt any complaints on what I quoted by saying that I’m not trying to press anything on anyone (I’m rather nominally Christian anyway) but am just answering the OP – What scriptures speak of premarital sex.