It's not all that hard, really. Most of us are human (note the 'most'
). Humankind will always try to survive.
That aside, you don't know a thing about your chances against the aliens. They could be anything from six foot tall sticklike insects who only figured out how to fly through space, bluff a litte and remove other creature's organs, to muscular, overly armed humanoids with impenetrable forcefield technlogy and whatever else.
Maybe there's a hivemind, maybe they are all intellignet. Too much speculation.
But, even assuming the aliens are the well equipped type, the moral thing for humans to do is to fight. The results will probably be a short but brutal massacre and a planet where the alien life is no longer possible. Maybe even a few dead aliens. What'll happen after that is that mankind is gone, and the aliens move on, maybe less in number but certainly not more.
If there's no fight, mankind will have a few more years before it's gone, the agressor stays on the planet a while and then moves off stronger than ever to plague another race.
And I don't think those last years will be too enjoyable either. Knowing aliens are after your world and have already treated you isn't pleasant, there's no guarantee they don't come down to kill you anyway for the fun of it, there's no future and, as a result, there'll soon be no laws. What these aliens promise had better be real good, or those final years won't be very happy at all.
Also, I don't believe the aliens have a right to deny my potential children life. If I had to make a choice between the life of a stranger coming with lethal intent and the life of my child, that wouldn't really be a choice, now would it? The situation the ETs put you in make that you don't have a lot to lose. If you've nothing to lose but everything to gain, that's all the more reason to fight.