Poppers are alkyl nitrites. Commercially available poppers these days are usually iso-propyl nitrite.
Iso-butyl nitrite has been linked to cancer in rats, and amyl nitrite is prescription only.
The effects of poppers last about 2 minutes.
One of the main effects is that they relax smooth muscle in the body. Smooth muscle includes lots of muscle which is not under voluntary control.
All this muscle goes slack and this results in a drop in blood pressure, as a result the heart goes faster in order to keep blood pressure up.
The slackness of the smooth muscle means that the muscle around the internal anal sphincter also goes slack, which means that it is easier to penetrate.
Muscle inside the vagina also slackens, which is reasonable because this is really the same group of muscles.
Apart from this the nitrogen in the vapours binds in the blood and cause rapturous feelings rather in the same way that inhaling nitrous does, with nitrogen bonds being formed in the blood and limiting oxygen supply.
The brain experiences this as a loss of inhibition (less control by the cortex) and the limbic system takes over.
Women having vaginal sex often enjoy poppers as much as men do.
The effects wear off very quickly, but be cautious after using as dizziness and fainting may occur if you get up too soon.
Needless to say, the effects are very much magnified if you have taken mdma or smoked weed.