Is the wetness a woman feels upon hearing the words "I love you" from her husband seminal fluid that necessitates ghusl, or is it another type of wetness that does not necessitate it?

The wetness you usually feel is madhy (pre-seminal fluid), which does not necessitate a full ritual bath (ghusl). Rather, it only requires ablution (wudu'), based on the Prophet's (peace and blessings of Allah be upon him) saying: "For it is ablution." Madhy is impure (najis), so the part of the body it touches must be washed. As for clothes stained by it, it is sufficient to take a handful of water and sprinkle it on the area you believe it has touched, based on his (peace and blessings of Allah be upon him) saying: "It is enough for you to take a handful of water and sprinkle your garment with it where you think it has touched."