You're getting a lot of bad advice in this thread from people who are very negative about marriage.

Even if you live together long term without getting married, there are likely to be legal and financial repercussions. As horrible as going through a divorce is, it's actually good to have the state (an impartial third party) involved in the disposition of assets. Breaking up after living together unmarried for 20 years has to be at least as nightmarish (if not moreso) than divorce, because the state will not step in to fairly divide your belongings. As much as people complain about divorce laws, the intent in writing those laws was to be fair and ensure that the less moneyed spouse would not suffer disproportionately from a failed marriage.

Go into marriage with your eyes open: understand the legal and financial ramifications, what it does and doesn't do, and what it can and cannot protect.

Generally, assets acquired or earned during marriage are considered marital property. So in your case, if you were to buy the house before marriage, you would retain the equity you built prior to marriage. Once you are married and you continue to build equity, your spouse would share equity with you 50/50. In the event of a divorce you'd keep your pre-marriage equity and you would buy out your spouse (giving them more cash in exchange for their equity, if you would like to keep the home.)

You could do a prenup, but there are many good reasons not to have one. Marriage is a partnership; you should try your best to share things fairly for the benefit of both individuals. If you go this route, I urge you to be fair. For example, stipulate in your agreement that in event of a divorce you would retain the right to purchase your partner's equity and keep the home.

How can an individual best safeguard their personal financial interests prior to marriage?

I think this question is the most important one you asked in your post, but no one is answering it. The answer is to honestly and transparently communicate about money with your prospective spouse. What is their philosophy toward money? Are they a spender or saver? What are their financial goals? Do they stick to a budget? How do they manage their money? What debts do they have, and what is their attitude toward debt? If you are living together, use a joint checking account to pay shared expenses, and keep your own spending and savings separate. Basically, marrying someone with a responsible attitude toward money (whose attitude toward money meshes well with your own) is probably the best way to safeguard your financial interests.