No individual food is "not healthy" it is your diet that overall is healthy or not. Using white flour instead of whole flour gives away a chance to eat some fibre. A page from the UK's NHS explains how much fibre you need and the role it plays in health.
In addition, your friends may object to wheat or flour (even whole wheat) because it is a carbohydrate. There is a popular belief that eating carbs will give you diabetes or cause you to gain weight. Refined carbohydrates, such as white flour, are considered even worse. The government of Canada advice certainly doesn't say "don't eat carbs" but I am sure that will not stop your friends from telling you not to eat them.
Diseases are not brought on directly by eating a specific food. Eating too much of one thing (especially if that leads to eating not enough of another) can, over time, contribute to the development of obesity, diabetes, heart conditions, and so on. A huge UK report concludes:
the evidence from both prospective cohort studies and randomised controlled trials indicates that total carbohydrate intake appears to be neither detrimental nor beneficial to cardio-metabolic health, colo-rectal health and oral health.
This includes sugars (also called simple carbohydrates) and starches (also called complex carbohydrates). It points out that sugars are linked to dental problems (cavities) and that eating unrefined grains (eg whole wheat, brown rice) is associated with a lower risk of diabetes and heart issues. It also emphasizes the importance of fibre. If you can't read the whole report, read pages 17 through 20 which are the recommendations.
With all that in mind, if you eat plenty of fruits and vegetables you can eat a little white bread from time to time (especially homemade bread where you know exactly what's in it) without worrying. Consider learning how to make whole wheat bread as well: it is also tasty and chewy and you might like it a lot. Don't worry about your friends; there will always be people telling you that "sugar is poison" or "we aren't designed to drink milk" - get your dietary advice from informed sources and you will have more confidence in what you cook and eat.