I think it was only his feet. Spreading his skirt over her meant extending his protection and intention to marry her if he could. There was still that other "near kinsman" who should have offered her protection but had not. I'm sure he may have wanted more, especially since he had been drinking wine; but I don't anything sexual happened since it says she lay at his feet. They hadn't done anything wrong so far as I can see; but Boaz was clever enough to want to avoid any appearance of evil, so he told her to sneak away before dawn. Maybe I'm prejudiced, but I like them both and want to believe the best. I also admire Naomi's cleverness.
It also depicts how male and female are joined together spiritually. I think we err if we think the "lower" position means "inferior." Both are required to be in the proper positions -- the female is closer to the earth so she is positioned between the earth and the feet of the man. Compare to the "fallen woman" anointing Jesus' feet with her hair.
In some ways, the "lower" position can be seen as the better one. I think Eve's error was to buy the serpent's idea that maybe she was inferior somehow -- and that idea is still with us! I suppose it will be with us as long as the struggle between the seed of the woman and seed of the serpent is with us.
Luke 14:11 For whosoever exalteth himself shall be abased; and he that humbleth himself shall be exalted.
Indeed, and we probably have enough to talk about already.
lol The customs of the Hebrews / Israelites are interesting....look into that if you are interested.