OLD fashioned preacher's post in investing was marked as the answer
First, let me address the financial difference between purchase of a product at either retail or wholesale level verses investment in a company or other organization via public market trade.
When a product that you need is offered as a purchasable commodity (water) you have some options (how many you have available has limitations such as number of vendors, distance to alternate venues, etc.). Your purchase does put money into the hands of someone who is going to use their wealth wickedly -- the city or county who handle the public water system, the company you bought bottled water from, the bottling plant that supplied your retailer with that water, the plastics manufacturer who made the bottle, the trucking firm and their employees who delivered the stock,etc. On this basis, your friend has a valid point. There probably doesn't exist a product which has a clean path from raw material to manufacturing to transportation to distribution to consumption.
On the other hand, publicly traded stocks and credit union account are both a purchase of ownership (which is why voting rights are conferred). If you buy enough ownership into the company you have a great (but not guarantied) chance of your vote changing some operational aspects. If you purchase 51% of the company (not usually offered into availability) you can now make the changes you want without effective opposition for you have "controlling rights". The average level of investment (such as you described) makes you a direct partner (as one of the 15,000 - or whatever number is applicable - owners) with no hope of effecting change.
Now, as for your wanting Scripture that applies to your predicament as outlined in the question above. The last phrase in Rom 14:23 ... for whatsoever is not of faith is sin.
IF you have to ask, justify and wonder THEN you're still not ready to act on it.
Mat 19:6 Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder. Mar 10:9 What therefore God hath joined together, let not man put asunder.
It doesn't matter who defines marriage or what they define it as --- If God doesn't join them, it's NOT marriage. The state's definition, approval or license doesn't make them married or not married. A preacher, priest, rabbi, justice of the peace or judge can "pronounce" anything he wants (as often as he wants) and if GOD doesn't join then together -- they're NOT married. Has nothing to do with the U.S. Constitution.
As to the U.S. Constitution and "separation". It was never intended to keep the church from having a voice in politics, it was designed to keep the government out of "religious" affairs (side note, the government didn't insert themselves [illegally] into marriage [licenses] until the 1800's).