A shocking message one teenager sends to another leads to scandal and death in an opera featured in this season’s “Live in HD” series from the Met. No, it’s not Two Boys, which premieres Monday: that work will not be included among the telecasts because of its “adult themes.” Eugene Onegin, on the other hand, is perfectly okay for children of all ages. Other child-appropriate themes the Met will explore this season include rape, dismemberment, adultery, suicideethnic cleansing and interspecies sexual relationships.