Nepal by far is very safe destination for travel for lone female travellers. There are very few cases or no cases at of of sexual harassment of any kind but however it is always advised to take some safety measures as a female travellers. Although nepal is a patriarchal society the only thing men see foreign woman as is as the tourists. Women travellers are skeptical about hitting the trail with male guide or how it would be being on the trail alone. The trails are safe and the only way people look at you is traveller.
We have had a reputation of having taken female travellers in groups as well as alone with male guide and there has absolutely never been any problem. It is advised to female travellers to clarify your limitations with the company, guides or any random people they hit on the road and people would respect your say.
Since the movies of west portray a different image of women in the communities, it is suggested not to wear revealing clothes. Look around you and adapt to the surrounding, wearing shorts and tank tops would not be a problem in the cities but however you may want to reconsider in the villages.
It cannot be said that you will not encounter sexual harrassments and specially when you plan to take public vehicles. It's not that people grope in public places as mentioned in many internationally written books but it is that the buses are crowded over the limit. In such circumstances where there is no space to breathe or even stand on 2 feet it is obvious that at times we have our nose under someone’s armpit. To avoid such circumstances it is advised to take a seat on the front compartment of the bus or stand in the same area. People would not hesitate to ask you questions like why aren't you married or why are you single with genuine concerns. To avoid such question having another trekking or hiking partner of any gender would do the trick. Nevertheless, it is not much of a concern.