Touching a woman's breast while kissing is for sexual gratification; don't con yourself into thinking otherwise. You're enjoying the sexual experience, but feel guilty that what you are experiencing is lust - and therefore a sin.
If your actions are done in love, respect and full consent, then what about it is sinful? Sexual desire is what creates babies, nomatter how religious you are. Sexual desire is as natural as the desire to eat or sleep. Take sexual desire away, and the human race would be extinct in one generation. One cannot even have sex without sexual desire. Think about it. So sexual desire cannot in itelf be a sin.
If you want to get technical about words, here's what lust means:
Main Entry:
1lust
Pronunciation: 'l&st
Function:
noun
Etymology: Middle English, from Old English; akin to Old High German
lust pleasure and perhaps to Latin
lascivus wanton
1 obsolete a : [size=-1]PLEASURE[/size], [size=-1]DELIGHT[/size] b : personal inclination
: [size=-1]WISH[/size]
2 : usually intense or unbridled sexual desire
: [size=-1]LASCIVIOUSNESS[/size]
3 a : an intense longing
: [size=-1]CRAVING[/size] b : [size=-1]ENTHUSIASM[/size], [size=-1]EAGERNESS[/size]
People try to whitewash sexuality into a very mechanical, non-personal, for procreation only type of thing. But sharing sexual intimacy with another person is a basic aspect of being human. The moral implications of sexuality is in how we treat others in the process, as wel as personal religious preferences and interpretations.
If lust is always wrong, then a man who has "an intense longing" for his wife is a sinner? How on earth does that make sense. Even "intense or unbridled sexual desire" can be expressed respectfully towards one's own spouse. "Unbridled" simply means "free from restraint" which can be vital to a healthy sex life, and how can "intense...sexual desire" necessarily be a sin in every circumstance?
Bottom line is this...the bible does not say sexual desire in itself is wrong, just the manner in which it is expressed. Having a sexual desire towards your girlfriend/fiance is natural, normal, and expected. Kissing your mother has no sexual desire involved, but every long kiss with your significant other necessarily will - so if sexual desire is a sin, who've crossed that line long ago. So don't be silly about this. Search your heart and be honest with yourself and your god. If you act accordingly, you'll be fine.