Touching anything that is normally covered by undergarments, will lead people into arousal, which will lead to further sexual temptation.
(Yes I know some internet contrarian is thinking..well what if someone doesn't wear undergarments..then what?? huh huh? then does anything go???--*sigh* Of course not. You get the main idea of what I'm getting at.)
We're directed by Jesus to pray, to not be lead into temptation.
When we arouse others and ourselves, we're negating the very command Jesus directs us to pray.
We're told by God not to take advantage of people sexually to meet our lusts or their lusts. When we do such we invite action from God upon us. The very God who gives us the Holy Spirit.
1 Thes 4
3 It is God’s will that you should be sanctified: that you should avoid sexual immorality;4 that each of you should learn to control your own body[a] in a way that is holy and honorable,5 not in passionate lust like the pagans, who do not know God;6and that in this matter no one should wrong or take advantage of a brother or sister.[b] The Lord will punish all those who commit such sins, as we told you and warned you before.7 For God did not call us to be impure, but to live a holy life.8 Therefore, anyone who rejects this instruction does not reject a human being but God, the very God who gives you his Holy Spirit.
These commands are for our own good.
Deut 10
12 And now, Israel, what does the Lord your God ask of you but to fear the Lord your God, to walk in obedience to him, to love him, to serve the Lord your God with all your heart and with all your soul,13 and to observe the Lord’s commands and decrees that I am giving you today for your own good?
The good news is that God has provided a way for us sinful humans to obey those commands.
It's through trusting 100% in what Jesus did, and not in trusting in our own power to will obedience.
2 Peter 1
3 His divine power has given us everything we need for a godly life through our knowledge of him who called us by his own glory and goodness.4 Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature, having escaped the corruption in the world caused by evil desires.
So when you see these commands, learn to love them because it is God's perfect way.
Then run to Jesus, because without him, there is no way you can fully obey.