I could be wrong, however, i think that it is for security reasons that Y passengers cant use the F bathroom. I believe that they dont want passengers congregating in the front of the plane as perhaps it could be interpreted as something else.
On a side note, I dont really care too much if Y passengers use the F bathroom if they are discreet about it. However, if they start congregating and/or being noisy, messy, and smelly, then it gets annoying.
However, i think that F passengers shouldnt be so arrogant in that regard. If a Y passenger REALLY has to "go" then let them do it.
Many times as a F passenger i go use the bathrooms in the back of Y cabin for several reasons. The main reason being to walk around and stretch my legs. The other reason is to be with "the people" as this allows me to appreciate my F seat and cabin even more.