@JimmyJ,
Because, from what I understand, thermodynamics teaches that in any closed system the amount of heat in that system will eventually reach equilibrium. So if at any given moment there must have been an infinite amount of time before that moment, and before that moment and so on, the only logical conclusion would be that if the universe existed for an infinite amount of time, then all areas in the universe would be equally warm. Obviously, we can tell this is not true, because we can observe differences in temperature.
It also teaches that no new energy can be created, it is only recycled into a less usable form of energy. Using the same infinite principle, all energy would have become benign an infinite amount of time ago.
A third understanding is that of course the universe is expanding and particles are drifting further and further apart. if this has been ongoing for an infinite amount of time, then our particles would be space dust, drifting ever after.
By my understanding, which could indeed be incorrect, to claim the universe is infinite, is to ignore basic scientific principles, which by your arguments for atheism I know is important to you.
On the second note, If something1 that operates outside of this universe caused nothing to become something2, then something1 would not be bound by the laws that bind the universe, and more specifically time. Something that is not bound by time would indeed be infinite, and not require a beginning.
You'll have to excuse me if I have misused science in any part of this statement.
Smiley,
not a scientist